fix: harden SQLite concurrency

[seonghobae]

Summary

• enable SQLite WAL and a 10-second busy timeout
• return retryable HTTP 503 for SQLite busy/locked errors
• serialize calendar create/delete/sync and profile deletion for the current single-worker deployment

Verification

• 89 tests passed
• statement coverage: 781/781 (100%)
• branch coverage: 184/184 (100%)
• 256 concurrent synthetic writes: 0 failures
• Ruff, JavaScript syntax, and container build passed

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Review details

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: cd022729-fbfa-4af0-b8c3-671ee688f3f1

📥 Commits

Reviewing files that changed from the base of the PR and between 3ad27ea and 053ae82.

📒 Files selected for processing (4)

• app/main.py
• app/store.py
• tests/test_api.py
• tests/test_store.py

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[seonghobae]

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