Serialize local sqlite initialization under a file lock

[anth-volk]

Fixes #3746

Summary

Stage 5's 50/50 LB test triggered the Cloud Run service's first real scale-out, and cold instances served brief 503 bursts: with WEB_CONCURRENCY=2, both gunicorn workers import the app concurrently on a fresh instance, both can pass the sqlite exists() check, and both run initialize() against the same file — the loser dies at boot with sqlite3.IntegrityError: UNIQUE constraint failed: policy.id (revision logs, 2026-07-08 20:07). A worse silent variant also exists: a worker can observe the file created but not yet seeded, skip initialization, and serve with empty reference tables.

Change

One block in data.py: the exists-check + initialize() now runs under an exclusive fcntl file lock — exactly one process initializes; the others block until it finishes, then see the complete database and skip. ~10 lines, local-sqlite path only; the remote (Cloud SQL) path is untouched.

Verification

Empirical 6-way concurrent fresh-boot test (fork model, mirroring gunicorn):

	Unfixed (3 rounds)	Fixed
All workers boot	❌ every round	✅
Policy rows seen across workers	{0, 2, 5} — including the silent empty-table variant	{5} exactly

Unit suite: 658 passed (one pre-existing local-env error in ai_prompts reproduces identically on unfixed master; CI arbitrates).

Context

This is the last known blocker class before ramp stages: any scale-out under real traffic would otherwise risk 503 bursts and silently mis-initialized workers. Stage 5's zero-5xx gate re-runs after this deploys.

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🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.92%. Comparing base (ab7ee0e) to head (05801c1).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3747      +/-   ##
==========================================
+ Coverage   79.89%   79.92%   +0.02%     
==========================================
  Files          70       70              
  Lines        4332     4338       +6     
  Branches      807      808       +1     
==========================================
+ Hits         3461     3467       +6     
  Misses        652      652              
  Partials      219      219              

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