feat(db): WAL-safe seismo_relay.db snapshot helper

sfm/db_snapshot.py

@@ -0,0 +1,24 @@
+"""
+db_snapshot.py — WAL-safe snapshot / restore helpers for seismo_relay.db and
+the waveform store, backing the /db/snapshot, /db/waveforms/recent.zip and
+/db/restore endpoints in server.py. Kept out of server.py so the logic is
+unit-testable without HTTP.
+"""
+from __future__ import annotations
+import sqlite3
+from pathlib import Path
+
+
+def snapshot_db(src_path: Path, dest_path: Path) -> Path:
+    """WAL-safe copy of a live SQLite DB via the SQLite online backup API.
+    Consistent even while the source is being written under WAL, and never
+    blocks the source's writers."""
+    src = sqlite3.connect(str(src_path))
+    dest = sqlite3.connect(str(dest_path))
+    try:
+        with dest:
+            src.backup(dest)
+    finally:
+        src.close()
+        dest.close()
+    return Path(dest_path)