fix: restore deleted j0k1 migration to keep released chain resolvable

[Devathmaj]

Description

Fixes a startup crash on existing databases. PR #15 deleted a released Alembic migration, j0k1l2m3n4o5 (schema_freeze_vendor_mappings), and rewired the chain to skip it. Any database that was stamped at that revision (it was the chain head between 2026-07-23 and 2026-08-13) can no longer resolve its current revision, so alembic upgrade head fails at app startup with:

alembic.util.exc.CommandError: Can't locate revision identified by 'j0k1l2m3n4o5'

Alembic migrations must never be deleted once released — the scripts directory and the DB's alembic_version table must always agree.

Changes:

1. Restored j0k1l2m3n4o5_schema_freeze_vendor_mappings.py with its original content. Its upgrade() is a safe no-op on fresh databases (h4d5e6f7a8b9 already creates vendor_mappings) and skips when the table exists, so re-adding it has no effect on new installs. Added a docstring NOTE that this released revision must never be deleted.
2. Rewired k2l3m4n5o6p7_add_notification_outbox.py back to down_revision = j0k1l2m3n4o5, restoring the original single linear chain with one head (m6n7o8p9q0r1). Also made its notification_outbox creation defensive (skips when the table already exists) so create_all-built databases upgrade cleanly.

The affected databases need no manual fix — re-running the app's startup migration now resolves from j0k1l2m3n4o5 to head automatically.

Type of Change

• Bug fix
• New feature
• New source
• Configuration / settings change
• Database migration
• Documentation update
• Refactor (no functional change)
• Other:

Affected Components

• Scheduler / Dispatcher
• HTTP Policy Layer (http_policy.py)
• RSS Collector
• Website Collector
• Reddit Integration
• AI Layer (Groq / Gemini)
• Email Notifications
• Database / Migrations
• API / Routers
• Configuration / Settings

Testing

• Ran pytest — all tests pass (359 passed, 15 skipped)
• Ran ruff check . && ruff format . — no lint errors
• Ran python scripts/verify_sources.py — all sources resolve (if sources were added or modified)
• Added unit tests for new business logic
• Added integration tests using fixtures or recorded responses (no live network calls)
• Mocked Reddit API client in all new tests
• Mocked Groq / Gemini responses in all new tests

Migration

• This PR includes an Alembic migration change
• The migration is safe to apply to a live database without downtime
• A sensible server-side default is provided for any new columns

Notes:
No new schema change — this restores a previously released revision so alembic upgrade head resolves on existing databases. The restored j0k1l2m3n4o5.upgrade() is a no-op (table already created by an earlier migration). The k2l3m4n5o6p7 outbox creation is now defensive against pre-existing tables from create_all-built databases.

Policy Checklist

• This PR touches one or more policy-sensitive files

AI Layer Changes

N/A — no prompt, schema, or provider logic changed.

Additional Notes

Verification: alembic history and alembic heads show a single linear chain with one head (m6n7o8p9q0r1); tests/test_migrations.py enforces the single-head/linear invariant and passes; full suite is green.