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/CONTEXT.md

@@ -0,0 +1,52 @@
+# Feature Context: Observability & Proxy Management
+
+## Configuration
+- **Development mode:** Automated
+- **Execution mode:** Orchestrator
+- **Strategy:** Incremental
+- **Build (full):** `make build`
+- **Build (frontend):** `cd web && npm install && npm run build`
+- **Build (backend):** `go build -o docker-watcher ./cmd/server`
+- **Test:** `go test ./...`
+- **Lint (backend):** `go vet ./...`
+- **Lint (frontend):** `cd web && npm run check`
+- **Dev server:** `make dev` (port: 8080)
+
+## Current State
+Feature branch just created. No implementation yet. Codebase is fully working on main.
+
+## Temporary Workarounds
+(none yet)
+
+## Cross-Phase Dependencies
+- Phases 2 & 3 depend on Phase 1 (schema, event_log table, store methods)
+- Phases 4, 5, 6, 7 depend on their respective backend phases (1-3) for API endpoints
+- Phase 8 depends on Phases 1-3 for backend infrastructure and event system
+
+## Deferred Work
+(none yet)
+
+## Failed Approaches
+(none yet)
+
+## Review Findings Log
+(none yet)
+
+## Phase Execution Log
+| Phase | Agent Used | Test Writer | Parallel | Notes |
+|-------|-----------|-------------|----------|-------|
+| (none yet) | | | | |
+
+## Environment & Runtime Notes
+- Build is currently blocked on Go 1.25 transitive dep from Docker SDK — may need to use Go 1.24 toolchain
+- SQLite has MaxOpenConns=1, so all DB operations are serialized
+- Frontend is embedded into Go binary via embed.FS
+
+## Implementation Notes
+- Event bus (`internal/events/bus.go`) uses buffered channels (64 cap), non-blocking publish
+- NPM client (`internal/npm/client.go`) handles JWT auth with auto-refresh
+- Store uses additive migrations — new `ALTER TABLE` statements are appended to runMigrations(), errors ignored for idempotency
+- New tables use `CREATE TABLE IF NOT EXISTS` in the schema constant
+- All API responses use envelope pattern: `{success: bool, data?: T, error?: string}`
+- Frontend types in `web/src/lib/types.ts` mirror Go models
+- API functions centralized in `web/src/lib/api.ts`