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_retry_replacement.py

@@ -1,6 +1,6 @@
 """Regression tests for /retry replacement semantics."""
 
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 
@@ -13,12 +13,10 @@ from gateway.session import SessionStore
 @pytest.mark.asyncio
 async def test_gateway_retry_replaces_last_user_turn_in_transcript(tmp_path):
     config = GatewayConfig()
-    with patch("gateway.session.SessionStore._ensure_loaded"):
-        store = SessionStore(sessions_dir=tmp_path, config=config)
-    store._db = None
-    store._loaded = True
+    store = SessionStore(sessions_dir=tmp_path, config=config)
 
     session_id = "retry_session"
+    store._db.create_session(session_id=session_id, source="test")
     for msg in [
         {"role": "session_meta", "tools": []},
         {"role": "user", "content": "first question"},