feat(parcel-sync): eTerra health check + maintenance detection

- New eterra-health.ts service: pings eTerra periodically (3min),
  detects maintenance (503, keywords), tracks consecutive failures
- New /api/eterra/health endpoint for explicit health queries
- Session route blocks login when eTerra is in maintenance (503 response)
- GET /api/eterra/session now includes eterraAvailable/eterraMaintenance
- ConnectionPill shows amber 'Mentenanță' state with AlertTriangle icon
  instead of confusing red error when eTerra is down
- Auto-connect skips when maintenance detected, retries when back online
- 30s session poll auto-detects recovery and re-enables auto-connect

src/modules/parcel-sync/services/session-store.ts

@@ -32,6 +32,12 @@ export type SessionStatus = {
   activeJobCount: number;
   /** First running job's phase (for UI hint) */
   activeJobPhase?: string;
+  /** eTerra platform health */
+  eterraAvailable?: boolean;
+  /** True if eTerra is detected as being in maintenance */
+  eterraMaintenance?: boolean;
+  /** Human-readable health message */
+  eterraHealthMessage?: string;
 };
 
 /* ------------------------------------------------------------------ */