fix: prevent PostgreSQL migrations from silently rolling back

[GeiserX]

Summary

• Fix PostgreSQL migrations silently rolling back due to advisory lock triggering SQLAlchemy's autobegin before context.configure(), causing Alembic to skip transaction management
• Switch from session-level pg_advisory_lock() to transaction-scoped pg_advisory_xact_lock() inside the begin_transaction() block

Root Cause

The advisory lock in alembic/env.py was acquired before context.configure(). This triggered SQLAlchemy's autobegin, which made Alembic detect _in_external_transaction=True and return nullcontext() from begin_transaction() -- skipping all commit logic. DDL ran but was rolled back on connection close.

Test plan

• Verify PostgreSQL migration 006 applies correctly on a fresh database
• Verify PostgreSQL migration is a no-op when already at version 006
• Verify SQLite migrations still work (no advisory lock path)
• Verify concurrent container starts don't deadlock (advisory lock still active)

Fixes #70