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

internal/store/store.go

@@ -81,6 +81,8 @@ func (s *Store) runMigrations() error {
 		`ALTER TABLE stages ADD COLUMN enable_proxy INTEGER NOT NULL DEFAULT 1`,
 		// Add ssl_certificate_id to settings (2026-03-29).
 		`ALTER TABLE settings ADD COLUMN ssl_certificate_id INTEGER NOT NULL DEFAULT 0`,
+		// Add stale_threshold_days to settings (2026-03-30).
+		`ALTER TABLE settings ADD COLUMN stale_threshold_days INTEGER NOT NULL DEFAULT 7`,
 	}
 
 	for _, m := range migrations {
@@ -98,6 +100,9 @@ func (s *Store) runMigrations() error {
 		`CREATE INDEX IF NOT EXISTS idx_stages_project_id ON stages(project_id)`,
 		`CREATE INDEX IF NOT EXISTS idx_stage_env_stage_id ON stage_env(stage_id)`,
 		`CREATE INDEX IF NOT EXISTS idx_volumes_project_id ON volumes(project_id)`,
+		`CREATE INDEX IF NOT EXISTS idx_event_log_severity ON event_log(severity)`,
+		`CREATE INDEX IF NOT EXISTS idx_event_log_source ON event_log(source)`,
+		`CREATE INDEX IF NOT EXISTS idx_event_log_created_at ON event_log(created_at)`,
 	}
 	for _, idx := range indexes {
 		if _, err := s.db.Exec(idx); err != nil {
@@ -250,6 +255,28 @@ CREATE TABLE IF NOT EXISTS volumes (
 	created_at TEXT NOT NULL DEFAULT (datetime('now')),
 	updated_at TEXT NOT NULL DEFAULT (datetime('now'))
 );
+
+CREATE TABLE IF NOT EXISTS event_log (
+	id         INTEGER PRIMARY KEY AUTOINCREMENT,
+	source     TEXT NOT NULL DEFAULT '',
+	severity   TEXT NOT NULL DEFAULT 'info',
+	message    TEXT NOT NULL DEFAULT '',
+	metadata   TEXT NOT NULL DEFAULT '{}',
+	created_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS standalone_proxies (
+	id                TEXT PRIMARY KEY,
+	domain            TEXT NOT NULL UNIQUE,
+	destination_url   TEXT NOT NULL DEFAULT '',
+	destination_port  INTEGER NOT NULL DEFAULT 0,
+	ssl_certificate_id INTEGER NOT NULL DEFAULT 0,
+	npm_proxy_id      INTEGER NOT NULL DEFAULT 0,
+	health_status     TEXT NOT NULL DEFAULT 'unknown',
+	health_checked_at TEXT NOT NULL DEFAULT '',
+	created_at        TEXT NOT NULL DEFAULT (datetime('now')),
+	updated_at        TEXT NOT NULL DEFAULT (datetime('now'))
+);
 `
 
 // Now returns the current time formatted for SQLite storage.