refactor(gateway): drop JSONL fallback in load_transcript

state.db is canonical. The 'use whichever source is longer' branch was
defensive code for the pre-DB migration; on every real DB it has not
fired (verified on a session corpus with 27 jsonl files / 950 sessions —
zero jsonl-bigger cases).

Test changes:
- TestLoadTranscriptCorruptLines: deleted (tested dead JSONL code path)
- TestLoadTranscriptPreferLongerSource: deleted (tested removed fallback)
- Replaced with TestLoadTranscriptDBOnly (DB-only reads)
- TestSessionStoreRewriteTranscript: fixture now creates DB session
- test_gateway_retry_replaces_last_user_turn: fixture uses real DB

tests/gateway/test_session_dm_thread_seeding.py

@@ -23,12 +23,9 @@ from gateway.session import SessionSource, SessionStore, build_session_key
 
 @pytest.fixture()
 def store(tmp_path):
-    """SessionStore with no SQLite, for fast unit tests."""
+    """SessionStore with SQLite — load_transcript reads from DB only."""
     config = GatewayConfig()
-    with patch("gateway.session.SessionStore._ensure_loaded"):
-        s = SessionStore(sessions_dir=tmp_path, config=config)
-    s._db = None
-    s._loaded = True
+    s = SessionStore(sessions_dir=tmp_path, config=config)
     return s