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/api/eventlog.go

@@ -0,0 +1,48 @@
+package api
+
+import (
+	"log/slog"
+	"net/http"
+	"strconv"
+
+	"github.com/alexei/docker-watcher/internal/store"
+)
+
+// listEventLog handles GET /api/events/log.
+// Supports query parameters: severity, source, since, until, limit, offset.
+func (s *Server) listEventLog(w http.ResponseWriter, r *http.Request) {
+	q := r.URL.Query()
+
+	limit, _ := strconv.Atoi(q.Get("limit"))
+	offset, _ := strconv.Atoi(q.Get("offset"))
+
+	filter := store.EventLogFilter{
+		Severity: q.Get("severity"),
+		Source:   q.Get("source"),
+		Since:    q.Get("since"),
+		Until:    q.Get("until"),
+		Limit:    limit,
+		Offset:   offset,
+	}
+
+	events, err := s.store.ListEvents(filter)
+	if err != nil {
+		slog.Error("failed to list events", "error", err)
+		respondError(w, http.StatusInternalServerError, "failed to list events")
+		return
+	}
+
+	respondJSON(w, http.StatusOK, events)
+}
+
+// getEventLogStats handles GET /api/events/log/stats.
+func (s *Server) getEventLogStats(w http.ResponseWriter, r *http.Request) {
+	stats, err := s.store.GetEventStats()
+	if err != nil {
+		slog.Error("failed to get event stats", "error", err)
+		respondError(w, http.StatusInternalServerError, "failed to get event stats")
+		return
+	}
+
+	respondJSON(w, http.StatusOK, stats)
+}