fix: open DuckDB read_only in trajectory helpers to avoid lock conflict with Streamlit

Both _load_window_ids and _load_mp_vectors_for_window only read from the DB.
Opening without read_only=True caused an IOException when Streamlit already held
a read-only lock, silently returning an empty scree plot.

analysis/trajectory.py

@@ -109,7 +109,7 @@ def _procrustes_align_windows(
 
 def _load_window_ids(db_path: str) -> List[str]:
     """Return all distinct window IDs from svd_vectors, in lexicographic order."""
-    conn = duckdb.connect(db_path)
+    conn = duckdb.connect(db_path, read_only=True)
     rows = conn.execute(
         "SELECT DISTINCT window_id FROM svd_vectors WHERE entity_type = 'mp' ORDER BY window_id"
     ).fetchall()
@@ -118,7 +118,7 @@ def _load_window_ids(db_path: str) -> List[str]:
 
 
 def _load_mp_vectors_for_window(db_path: str, window_id: str) -> Dict[str, np.ndarray]:
-    conn = duckdb.connect(db_path)
+    conn = duckdb.connect(db_path, read_only=True)
     rows = conn.execute(
         "SELECT entity_id, vector FROM svd_vectors WHERE window_id = ? AND entity_type = 'mp'",
         (window_id,),