fix(db): harden sqlite pragmas and dispose engine cleanly

[mojomast]

Summary

• apply SQLite runtime pragmas on each connection (WAL, synchronous=NORMAL, temp_store=MEMORY, foreign_keys=ON, configured busy_timeout)
• add SQLITE_BUSY_TIMEOUT_MS support and set engine connect timeout accordingly for better write contention behavior
• split DB runtime init to return engine + session maker and dispose the engine during app shutdown
• document new SQLite timeout setting in .env.example and README.md

Verification

• PYTHONPATH=src .venv/bin/python -m py_compile src/proxy_app/db.py src/proxy_app/main.py tests/test_db_runtime.py
• PYTHONPATH=src .venv/bin/pytest -q tests/test_db_runtime.py

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Important

Enhance SQLite handling by applying runtime pragmas, introducing a configurable busy timeout, and ensuring clean engine disposal.

• SQLite Configuration:
  • Apply runtime pragmas (WAL, synchronous=NORMAL, temp_store=MEMORY, foreign_keys=ON, busy_timeout) on each connection in db.py.
  • Introduce SQLITE_BUSY_TIMEOUT_MS environment variable for configuring SQLite busy timeout.
• Database Initialization:
  • Split DB runtime initialization in db.py to return engine and session maker separately.
  • Dispose of the engine during app shutdown in main.py.
• Documentation:
  • Document SQLITE_BUSY_TIMEOUT_MS in .env.example and README.md.
• Testing:
  • Add test_sqlite_pragmas_are_applied in test_db_runtime.py to verify SQLite pragmas are correctly applied.

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[mojomast]

Superseded by updates pushed to #123 to keep all MVP changes in one review thread.