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.

docs/operations/trivy-setup.mdx

@@ -44,7 +44,7 @@ When a newer Trivy release is available, Settings → Security shows an **Update
 
 To update automatically instead, toggle **Auto-update Trivy** on. Sencho checks for new releases once a day and installs them in the background. You'll get an in-app notification each time a new version is installed, or when an update is available and auto-update is off.
 
-The install, update, and uninstall buttons are Admiral-only. Skipper and Community instances see the scanner status, but install actions require an Admiral license.
+The install, update, and uninstall buttons are available to admins on every tier. The **Auto-update Trivy** toggle is Admiral only.
 
 ### Removing the managed install
 
@@ -212,11 +212,9 @@ This means the host binary is not ABI-compatible with the Sencho image. Use the
 
 The first scan after a Trivy install downloads the vulnerability database. Expect 10 to 30 seconds of additional latency. Subsequent scans are near-instant once the cache is warm and `TRIVY_CACHE_DIR` is persisted.
 
-### Install button is disabled
+### Install button is hidden
 
-The install, update, and uninstall buttons require an Admiral license. If you hold a Skipper or Community license, the card shows current status only; use Option 2 or 3 to add Trivy manually.
-
-The install button is also hidden when a host-installed Trivy is already detected on `PATH`. Remove the host binary (or drop the bind mount) to switch to the managed install.
+The install button is hidden when a host-installed Trivy is already detected on `PATH`. Remove the host binary (or drop the bind mount) to switch to the managed install. The button also requires the admin role; viewer accounts see the scanner status only.
 
 ### Private registry images fail to scan