feat: open security basics, manual fleet ops, and basic fleet management to Community (#930)

Realign tier guards to the user-stated philosophy: Community covers
deploy/monitor at scale plus security basics, Skipper adds automation
and advanced fleet management, Admiral keeps enterprise control.

Community now includes:
- Trivy install / uninstall / update from the Settings Hub (admin role)
- CVE suppressions CRUD (admin role; replicates fleet-wide)
- Manual image scan with vuln, secret, and misconfig results
- Stack-config scan, scan comparison
- Manual fleet snapshots: create, list, view, restore, delete
- Per-node Sencho self-update (Check Updates + per-node Update)
- Fleet Overview search, sort, filters, node-card expand, auto-refresh

Stays paid:
- Scan policies with block_on_deploy enforcement (Skipper+)
- SBOM (SPDX, CycloneDX), SARIF export (Skipper+)
- Bulk Update All across the fleet (Skipper+)
- Scheduled snapshot create (now Skipper, was Admiral)
- Trivy auto-update toggle, fleet-wide policy push (Admiral)

The Settings -> Security tab is unhidden by setting the registry tier to
null. The SecuritySection no longer early-returns a PaidGate; the policy
list, Add Policy button, and policy dialogs are wrapped in {isPaid && }.
The Fleet view drops isPaid gates on the Snapshots tab, Check Updates
button, per-node update handlers, OverviewToolbar grid controls, the
NodeCard expand affordance, and the auto-refresh notice. The
NodeUpdatesSheet receives a canBulkUpdate prop and gates the Update All
button on it. useFleetUpdateStatus and useFleetPolling drop their isPaid
guards so polling runs for Community; useFleetOverview drops the isPaid
wrap on the filter and sort path.

Backend route guards are flipped per the matrix above. The scheduler
tick and requireScheduledTaskTier add 'snapshot' to the Skipper+ branch.
Backend test assertions are inverted for the now-Community endpoints
and a positive Skipper-snapshot-task test is added.

Documentation across features/, api-reference/, and operations/ is
updated to reflect the new tier mapping.

backend/src/middleware/tierGates.ts

@@ -59,9 +59,9 @@ export const requireNodeProxy = (req: Request, res: Response): boolean => {
   return true;
 };
 
-/** Tier gate for scheduled tasks: `update` and `scan` require Skipper+, everything else requires Admiral. */
+/** Tier gate for scheduled tasks: `update`, `scan`, and `snapshot` require Skipper+, everything else requires Admiral. */
 export const requireScheduledTaskTier = (action: string, req: Request, res: Response): boolean => {
-  if (action === 'update' || action === 'scan') return requirePaid(req, res);
+  if (action === 'update' || action === 'scan' || action === 'snapshot') return requirePaid(req, res);
   return requireAdmiral(req, res);
 };