fix(db): close SQLite connections explicitly to suppress ResourceWarning on Python 3.13

`sqlite3.Connection.__exit__` only commits/rolls back a transaction — it
does not close the connection. Python 3.13 (the Nix env on the KVM
runner) emits `ResourceWarning: unclosed database` for every connection
GC'd without an explicit close, producing noisy output in the coverage job.

Add `DbStore._connection()`, a `contextmanager` that calls `self._connect()`,
wraps it in the existing transaction context manager, and closes the
connection in a `finally` block. Change all `with self._connect() as conn:`
call sites in `db_store.py`, `audit_store.py`, `queue_store.py`, and
`orchestrator/registry.py` to `with self._connection() as conn:`.
`_connect()` remains as the per-subclass hook (RegistryStore overrides
it to set `busy_timeout`); `_connection()` delegates to `self._connect()` so
the override is respected.

bot_bottle/queue_store.py

@@ -66,7 +66,7 @@ class QueueStore(DbStore):
         super().__init__(resolved, migrations)
 
     def write_proposal(self, proposal: Proposal) -> Path:
-        with self._connect() as conn:
+        with self._connection() as conn:
             conn.execute(
                 """
                 INSERT OR REPLACE INTO supervise_proposals (
@@ -89,7 +89,7 @@ class QueueStore(DbStore):
         return self.db_path
 
     def read_proposal(self, proposal_id: str) -> Proposal:
-        with self._connect() as conn:
+        with self._connection() as conn:
             row = conn.execute(
                 """
                 SELECT * FROM supervise_proposals
@@ -104,7 +104,7 @@ class QueueStore(DbStore):
     def list_pending_proposals(self) -> list[Proposal]:
         if not self.db_path.is_file():
             return []
-        with self._connect() as conn:
+        with self._connection() as conn:
             rows = conn.execute(
                 """
                 SELECT p.* FROM supervise_proposals p
@@ -125,7 +125,7 @@ class QueueStore(DbStore):
     def list_all_pending_proposals(self) -> list[Proposal]:
         if not self.db_path.is_file():
             return []
-        with self._connect() as conn:
+        with self._connection() as conn:
             rows = conn.execute(
                 """
                 SELECT p.* FROM supervise_proposals p
@@ -142,7 +142,7 @@ class QueueStore(DbStore):
         return [self._row_to_proposal(row) for row in rows]
 
     def write_response(self, response: Response) -> Path:
-        with self._connect() as conn:
+        with self._connection() as conn:
             conn.execute(
                 """
                 INSERT OR REPLACE INTO supervise_responses (
@@ -161,7 +161,7 @@ class QueueStore(DbStore):
         return self.db_path
 
     def read_response(self, proposal_id: str) -> Response:
-        with self._connect() as conn:
+        with self._connection() as conn:
             row = conn.execute(
                 """
                 SELECT * FROM supervise_responses
@@ -176,7 +176,7 @@ class QueueStore(DbStore):
     def archive_proposal(self, proposal_id: str) -> None:
         if not self.db_path.is_file():
             return
-        with self._connect() as conn:
+        with self._connection() as conn:
             conn.execute(
                 """
                 UPDATE supervise_proposals SET archived = 1