feat(observability): phase 1 - schema, models & event log backend

Add database foundation for observability features:
- event_log table with severity/source filtering and pagination
- standalone_proxies table for user-created reverse proxies
- stale_threshold_days setting (default 7 days)
- Auto-persist warn/error events from event bus to database
- SSE broadcast of persistent events for real-time UI updates
- Frontend types and API functions for downstream UI phases

plans/observability-proxy-mgmt/phase-5-stale-ui.md

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+# Phase 5: Stale Containers UI
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** frontend
+
+## Objective
+Build the stale containers dashboard widget and dedicated view, with cleanup actions and settings configuration.
+
+## Tasks
+
+- [ ] Task 1: Add API functions in api.ts: fetchStaleContainers, cleanupStaleContainer, bulkCleanupStaleContainers
+- [ ] Task 2: Create StaleContainerCard component — shows: container name, project, stage, image tag, last alive timestamp, "X days stale" badge (color-coded by severity)
+- [ ] Task 3: Create stale containers section on dashboard (+page.svelte) — count badge, mini-list of top 5 offenders, "View all" link
+- [ ] Task 4: Create dedicated route `/containers/stale` with full stale container list
+- [ ] Task 5: Individual cleanup action — ConfirmDialog with warning, calls cleanup API
+- [ ] Task 6: Bulk cleanup action — "Clean up all" button with confirmation, progress indicator
+- [ ] Task 7: Settings integration — add stale_threshold_days field to settings page with validation (min 1 day)
+- [ ] Task 8: Add navigation link or sub-nav for stale containers
+- [ ] Task 9: Add i18n keys for stale containers
+
+## Files to Modify/Create
+- `web/src/lib/api.ts` — Add stale container API functions
+- `web/src/lib/types.ts` — Add StaleContainer interface
+- `web/src/lib/components/StaleContainerCard.svelte` — NEW: Stale container display
+- `web/src/routes/+page.svelte` — Add stale containers dashboard widget
+- `web/src/routes/containers/stale/+page.svelte` — NEW: Dedicated stale view
+- `web/src/routes/containers/stale/+page.ts` — NEW: Data loader
+- `web/src/routes/settings/+page.svelte` — Add stale threshold setting field
+- `web/src/routes/+layout.svelte` — Add nav link if needed
+
+## Acceptance Criteria
+- Dashboard shows stale container count and top offenders
+- Dedicated page lists all stale containers with details
+- Individual cleanup removes container with confirmation
+- Bulk cleanup works with progress feedback
+- Settings page allows configuring stale threshold
+- Severity coloring: 7-14 days = yellow, 14+ days = red
+- Responsive layout
+
+## Notes
+- Reuse existing ConfirmDialog for destructive actions
+- Dashboard widget should not slow down initial page load (lazy load or small payload)
+- Stale container data comes from GET /api/containers/stale (Phase 2)
+- Settings update uses existing PUT /api/settings endpoint
+
+## Review Checklist
+- [ ] All tasks completed
+- [ ] Code follows project conventions
+- [ ] No unintended side effects
+- [ ] Build passes
+- [ ] Tests pass (new + existing)
+
+## Handoff to Next Phase
+<!-- Filled in by the implementation agent after completing this phase. -->