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/models.go

@@ -55,8 +55,9 @@ type Settings struct {
 	WebhookSecret    string `json:"webhook_secret"`
 	PollingInterval  string `json:"polling_interval"`
 	BaseVolumePath   string `json:"base_volume_path"`
-	SSLCertificateID int    `json:"ssl_certificate_id"`
-	UpdatedAt        string `json:"updated_at"`
+	SSLCertificateID   int    `json:"ssl_certificate_id"`
+	StaleThresholdDays int    `json:"stale_threshold_days"`
+	UpdatedAt          string `json:"updated_at"`
 }
 
 // Instance represents a running (or stopped) container for a project stage.
@@ -117,3 +118,27 @@ type Volume struct {
 	CreatedAt string `json:"created_at"`
 	UpdatedAt string `json:"updated_at"`
 }
+
+// EventLog represents a persistent event log entry.
+type EventLog struct {
+	ID        int64  `json:"id"`
+	Source    string `json:"source"`
+	Severity string `json:"severity"` // info, warn, error
+	Message  string `json:"message"`
+	Metadata string `json:"metadata"` // JSON-encoded structured data
+	CreatedAt string `json:"created_at"`
+}
+
+// StandaloneProxy represents a standalone reverse proxy not tied to a project.
+type StandaloneProxy struct {
+	ID               string `json:"id"`
+	Domain           string `json:"domain"`
+	DestinationURL   string `json:"destination_url"`
+	DestinationPort  int    `json:"destination_port"`
+	SSLCertificateID int    `json:"ssl_certificate_id"`
+	NpmProxyID       int    `json:"npm_proxy_id"`
+	HealthStatus     string `json:"health_status"` // unknown, healthy, unhealthy
+	HealthCheckedAt  string `json:"health_checked_at"`
+	CreatedAt        string `json:"created_at"`
+	UpdatedAt        string `json:"updated_at"`
+}