Fix binding token extraction and harden startup concurrency

app/models/db.py

@@ -6,6 +6,8 @@ from sqlalchemy.orm import DeclarativeBase
 
 from app.config import Settings
 
+SCHEMA_COMPATIBILITY_LOCK_KEY = 2026030501
+
 
 class Base(DeclarativeBase):
     pass
@@ -62,6 +64,10 @@ async def ensure_schema_compatibility() -> None:
         "CREATE INDEX IF NOT EXISTS idx_token_bindings_ip ON token_bindings(bound_ip)",
     ]
     async with engine.begin() as connection:
+        await connection.execute(
+            text("SELECT pg_advisory_xact_lock(:lock_key)"),
+            {"lock_key": SCHEMA_COMPATIBILITY_LOCK_KEY},
+        )
         for statement in statements:
             await connection.execute(text(statement))