perf(db): enable WAL and busy timeout for SQLite

The generation worker and request handlers race on the database —
enough that orphan-recovery code exists in two places to clean up
after 'database is locked' failures. WAL lets readers proceed during a
write, synchronous=NORMAL is the recommended pairing, and the 30s
sqlite3 timeout waits on a locked database instead of raising
immediately.

backend/database/session.py

@@ -3,20 +3,20 @@
 import logging
 import uuid
 
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, event
 from sqlalchemy.orm import sessionmaker
 
 from .. import config
+from .migrations import run_migrations
 from .models import (
-    Base,
     AudioChannel,
+    Base,
     EffectPreset,
     Generation,
     GenerationVersion,
     ProfileChannelMapping,
     VoiceProfile,
 )
-from .migrations import run_migrations
 from .seed import backfill_generation_versions, seed_builtin_presets
 
 logger = logging.getLogger(__name__)
@@ -36,9 +36,22 @@ def init_db() -> None:
 
     engine = create_engine(
         f"sqlite:///{_db_path}",
-        connect_args={"check_same_thread": False},
+        # timeout is sqlite3's busy handler: wait up to 30s on a locked
+        # database instead of raising "database is locked" immediately.
+        connect_args={"check_same_thread": False, "timeout": 30},
     )
 
+    @event.listens_for(engine, "connect")
+    def _set_sqlite_pragmas(dbapi_connection, _connection_record):
+        # WAL lets readers proceed while a writer holds the lock, which is
+        # the main source of lock racing between the generation worker and
+        # request handlers. synchronous=NORMAL is the recommended pairing
+        # (durable across app crashes, fsyncs only on checkpoint).
+        cursor = dbapi_connection.cursor()
+        cursor.execute("PRAGMA journal_mode=WAL")
+        cursor.execute("PRAGMA synchronous=NORMAL")
+        cursor.close()
+
     SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
 
     run_migrations(engine)
@@ -47,7 +60,7 @@ def init_db() -> None:
     # Create default audio channel if it doesn't exist
     db = SessionLocal()
     try:
-        default_channel = db.query(AudioChannel).filter(AudioChannel.is_default == True).first()
+        default_channel = db.query(AudioChannel).filter(AudioChannel.is_default).first()
         if not default_channel:
             default_channel = AudioChannel(
                 id=str(uuid.uuid4()),