feat: apply SQLite WAL pragmas and a writer/readonly pool

[numdinkushi]

Commit Type

• feat: A new feature
• fix: A bug fix
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (formatting, etc.)
• refactor: A code change that neither fixes a bug nor adds a feature
• perf: A code change that improves performance
• test: Adding missing or correcting existing tests
• build: Changes affecting the build system or external dependencies
• ci: Changes to CI/CD configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit

Breaking change? (If yes, add BREAKING CHANGE in the commit footer)

• Yes — this contains a breaking change
• No

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What changed

SQLite init now applies WAL, busy_timeout=5000, and cache_size=-64000 from backend/src/services/sqlite/. initDb() opens one writer (getDb()) plus a readonly reader (getReadDb(), SQLITE_READ_POOL_SIZE, max 4). In-memory databases skip extra readers.

Tuning is recorded in docs/adr/0006-sqlite-wal-and-pool-tuning.md.

Existing getDb() call sites are unchanged.

Testing done

• npx vitest run src/services/sqlite src/db/concurrency.test.ts — 12 passed
  • WAL + busy_timeout + cache_size on writer and reader
  • reader snapshot during an open write transaction
  • busy_timeout waits on a lock instead of failing immediately
  • initDb() applies the same pragmas
• npx eslint src/services/db.ts src/services/sqlite src/db/concurrency.test.ts

Related issues

Closes #731

Checklist

• I used the correct commit type (see above) for the squashed commit message.
• I kept the change focused on the related issue.
• I added or updated tests where useful.
• I updated documentation where behavior changed.
• I verified the app still builds or explained why verification was skipped.
• My commits follow Conventional Commits (enforced by commitlint).

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