feat(admin): add admin backend endpoints and service layer

Why:
- Dashboard needs system health, Qdrant management, chunk preview, questions, and cost estimation endpoints

Changes:
- Add admin router with 11 endpoints (health, Qdrant CRUD, chunk preview, questions management, cost estimation)
- Add delete_experiment to SQLite storage
- Mount admin router and dashboard static files at /app

Impact:
- New /admin/* API routes available
- Dashboard served at /app/ via StaticFiles

src/storage/sqlite.py

@@ -311,6 +311,17 @@ def list_experiments(
         conn.close()
 
 
+def delete_experiment(experiment_id: str) -> bool:
+    """Delete an experiment by ID."""
+    conn = _connect()
+    try:
+        cur = conn.execute("DELETE FROM experiments WHERE id = ?", (experiment_id,))
+        conn.commit()
+        return cur.rowcount > 0
+    finally:
+        conn.close()
+
+
 # ── Internal helpers ───────────────────────────────────────────────
 
 _JSON_FIELDS = {"chunk_counts", "retrieved_chunks", "latency_breakdown",