feat: Add migration gate middleware and demo data seeding

[rsalus]

Fix startup race condition where dashboard queries hit prior_auth_requests
before migration completes (503 errors). Add MigrationGateMiddleware that
returns 503 with Retry-After until all migrations complete, allowing health
checks through. Add IDataSeeder interface and PARequestDataSeeder
that seeds 4 demo PA requests in development. Port patterns from aegis-api.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch 02-21-feat_add_migration_gate_middleware_and_demo_data_seeding

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[rsalus]

• feat: Add migration gate middleware and demo data seeding #30 👈 (View in Graphite)
• main

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[rsalus]

Merge activity

• Feb 22, 12:53 AM UTC: @rsalus merged this pull request with Graphite.