Audit-hardening pass for secret and misconfiguration scanning (#977)

* fix(security): dedupe concurrent compose-stack scans

Track stack scans in scanningImages keyed stack:<nodeId>:<stackName>.
The /scan/stack route returns 409 when an in-flight scan exists, and
the service-side check is the real correctness barrier (the route
pre-check is a fast-path optimization that mirrors scanImage). The
dedup key release lives in a try/finally so failed scans free the
slot for retry.

Why: scanComposeStack had no equivalent of scanImage's scanningImages
guard, so two simultaneous calls for the same stack would both run
trivy config, both insert a vulnerability_scans row, and double-
process the result.

* feat(security): acknowledge misconfig findings

Adds a parallel acknowledgement system for Trivy misconfig findings
that mirrors cve_suppressions: a new misconfig_acknowledgements table,
read-time enrichment via the new misconfig-ack-filter utility, REST
CRUD endpoints, fleet-sync replication from control to replicas, a
Settings panel, and an Acknowledge button on the Misconfigs tab.

Schema and behavior parity with cve_suppressions:
  - UNIQUE(rule_id, COALESCE(stack_pattern, '')) so fleet-wide acks
    collide as expected
  - blockIfReplica on every write
  - Audit-log entries name the scope (rule_id, stack_pattern) but
    never the reason text
  - replicated_from_control flag controls UI delete affordance and
    drives clearReplicatedRows on demote/reanchor
  - Validators reused: validateStackPatternForRedos for glob safety,
    sanitizeForLog for log fragments

SARIF export emits an external/accepted suppression entry per
acknowledged misconfig, matching the CVE pattern.

Per-row Acknowledge dialog prefills stack_pattern with the scan's
stack_context so the default scope is "rule + this stack only" and an
operator must broaden explicitly.

Tests: misconfig-ack-filter (15) and misconfig-ack-routes (23)
including the duplicate-409 case for both pinned and fleet-wide acks.

* fix(security): reap orphaned trivy tmp dirs at startup

When the buildEnv path writes a per-scan DOCKER_CONFIG dir under
os.tmpdir() and the process crashes before the finally block runs,
the dir leaks. Mirrors GitSourceService.sweepStaleTempDirs:
exported sweepStaleTrivyTempDirs is fire-and-forget at boot,
removes prefix-matching dirs older than 1 hour, swallows
permission/race failures, logs a single line if any were reaped.

* perf(security): emit per-batch summary for scanAllNodeImages

Adds one diag() line at the end of scanAllNodeImages summarising
unique image count, scanned, skipped, failed, violation count, and
elapsed time. Per-image diag inside scanImage stays useful for
debugging individual scans; the summary gives operators a single
fleet-level checkpoint when developer_mode is on.

* perf(security): cap SARIF export at 5000 findings per type

Replace the unbounded fetchAllPages walk on /scans/:id/sarif with a
hard limit of 5000 findings per type. When any type trips the cap,
emit run-level properties.truncated=true plus row_limit and per-type
totals so downstream tooling can flag the export as partial.
Console-warns for ops visibility.

A scan with 50k vulns previously streamed every row into memory
before serialising; the cap bounds memory and serialisation time at
the cost of completeness on pathological scans.

* docs(env): document TRIVY_BIN host-binary override

The env var is honored by TrivyService.detectTrivy as a fallback when
no managed install is present, but it was undocumented in
.env.example. Adds the var with a comment explaining precedence
(managed > TRIVY_BIN > PATH).

* test(security): cover scanComposeStack failure modes

Two new cases drive the existing try/catch through real failure
paths:
  - Malformed Trivy stdout: row flips to status='failed' with the
    parser error preserved on `error`.
  - execFile rejection: row flips to status='failed' with a string
    error message.

Pairs with the existing dedup tests so the failure path now also
verifies the scan row state, not just the thrown exception.

* test(e2e): security scanner + misconfig acknowledgement flow

Seven Playwright tests covering the scanner UI and the new
acknowledgement system end-to-end:
  - Trivy availability gate (skips suite when binary absent so CI
    without Trivy can opt out via E2E_SKIP_TRIVY=1)
  - Stack config scan completes and records misconfig findings
  - Concurrent stack scan returns 409 from the dedup gate
  - Misconfig ack POST creates and lists on Settings
  - Duplicate (rule_id, stack_pattern) returns 409
  - Malformed rule_id (shell metacharacters) returns 400
  - Misconfigs tab renders against a real stack scan

Tests drive the API for behaviour assertions and the UI only for
shell-rendering checks; the visual snapshot suite owns screenshots.

* docs(features): add misconfig acknowledgement workflow and SARIF cap

Refreshes vulnerability-scanning.mdx with:
  - Misconfig acknowledgements section covering the per-row dialog,
    Settings panel, scope/matching rules, and SARIF emission
  - Tier table row for the new feature
  - SARIF section note on the 5000 row-per-type cap and the
    properties.truncated marker for partial exports
  - Troubleshooting entries: SARIF cap, hidden Acknowledge button,
    findings resurfacing after delete, Trivy DB phone-home, and
    409 on concurrent compose-stack scans

* fix(ci): clear backend lint and CodeQL alerts

- Remove the dead fetchAllPages helper in routes/security.ts. It lost
  its callers when the SARIF endpoint switched to direct paged reads
  for the truncation cap. ESLint flagged it as unused.
- Switch the trivy-tmp-cleanup test helper to fs.mkdtempSync. Building
  paths under os.tmpdir() with predictable names tripped CodeQL's
  js/insecure-temporary-file rule (high severity), which warns about
  symlink-pre-creation attacks even in test code. mkdtempSync appends
  a process-random suffix and creates the dir atomically; the
  sencho-trivy- prefix is preserved so the production sweep still
  matches the test fixtures.

backend/src/routes/fleet.ts

@@ -93,6 +93,24 @@ function validateCveSuppressionRow(row: unknown): string | null {
   return null;
 }
 
+function validateMisconfigAcknowledgementRow(row: unknown): string | null {
+  if (!row || typeof row !== 'object') return 'row must be an object';
+  const r = row as Record<string, unknown>;
+  if (typeof r.rule_id !== 'string' || r.rule_id.length === 0 || r.rule_id.length > 200) return 'rule_id must be a non-empty string up to 200 chars';
+  if (r.stack_pattern !== null && typeof r.stack_pattern !== 'string') return 'stack_pattern must be a string or null';
+  if (typeof r.stack_pattern === 'string') {
+    if (r.stack_pattern.length > 300) return 'stack_pattern is too long';
+    const patternError = validateStackPatternForRedos(r.stack_pattern);
+    if (patternError) return patternError;
+  }
+  if (typeof r.reason !== 'string') return 'reason must be a string';
+  if (r.reason.length > 2000) return 'reason is too long';
+  if (typeof r.created_by !== 'string' || r.created_by.length > 200) return 'created_by must be a string';
+  if (typeof r.created_at !== 'number') return 'created_at must be a number';
+  if (r.expires_at !== null && typeof r.expires_at !== 'number') return 'expires_at must be a number or null';
+  return null;
+}
+
 interface FleetNodeOverview {
   id: number;
   name: string;
@@ -327,7 +345,11 @@ fleetRouter.get('/role', authMiddleware, (req: Request, res: Response): void =>
 fleetRouter.post('/sync/:resource', authMiddleware, (req: Request, res: Response): void => {
   if (!requireNodeProxy(req, res)) return;
   const resource = req.params.resource;
-  if (resource !== 'scan_policies' && resource !== 'cve_suppressions') {
+  if (
+    resource !== 'scan_policies'
+    && resource !== 'cve_suppressions'
+    && resource !== 'misconfig_acknowledgements'
+  ) {
     res.status(400).json({ error: `Unsupported sync resource: ${resource}` });
     return;
   }
@@ -353,7 +375,12 @@ fleetRouter.post('/sync/:resource', authMiddleware, (req: Request, res: Response
     res.status(413).json({ error: `Too many rows (max ${MAX_SYNC_ROWS})` });
     return;
   }
-  const validator = resource === 'scan_policies' ? validateScanPolicyRow : validateCveSuppressionRow;
+  const validator =
+    resource === 'scan_policies'
+      ? validateScanPolicyRow
+      : resource === 'cve_suppressions'
+        ? validateCveSuppressionRow
+        : validateMisconfigAcknowledgementRow;
   for (let i = 0; i < rows.length; i++) {
     const err = validator(rows[i]);
     if (err) {