feat(db): add Drizzle ORM with local SQLite storage

Configure Bun SQLite with WAL mode, foreign keys, and a shared database path for development and production.

Add migration commands and startup migration handling, check database connectivity in the health endpoint, and document configuration while excluding local database files from Git.

src/db/schema.ts

@@ -0,0 +1,2 @@
+// Define application tables here with drizzle-orm/sqlite-core.
+export {};