fix(db): declare transaction() on local Database interface

[lukeboyett]

Summary

The cross-runtime Database interface in src/db.ts declares the subset of
better-sqlite3 / bun:sqlite methods QMD uses. Commit fee576b ("fix: migrate
legacy lowercase paths on reindex") introduced a db.transaction(...) call in
src/store.ts:2142 but didn't extend the interface, so tsc -p tsconfig.build.json
fails on main:

src/store.ts(2142,22): error TS2339: Property 'transaction' does not exist on type 'Database'.

Both underlying engines expose transaction(fn), so this one-line addition just
makes the type reflect reality.

Change

 export interface Database {
   exec(sql: string): void;
   prepare(sql: string): Statement;
   loadExtension(path: string): void;
+  transaction<T extends (...args: any[]) => any>(fn: T): T;
   close(): void;
 }

Validation

• npm run build is green with this change, red without it (reproduced on current main at cfd640e).
• No runtime behavior change — pure type declaration on an interface that already matched both engines in practice.
• Codex Cloud separately reproduced the same TS2339 on main and flagged it as a build-blocker.

Test plan

• npm run build passes
• npm test — no new failures vs. main baseline