feat(db): verify database schema matches code on startup (#352)

[sparkysolutions10-pixel]

Closes #352

Summary of Changes

• Implements assertSchemaValid() in src/db/schemaVerify.ts checking for existence of all required tables and columns against PostgreSQL information_schema on boot.
• Fails fast and throws DatabaseSchemaMismatchError explicitly naming any missing tables or columns before request handling begins.
• Integrated into src/index.ts immediately after pgPool.connect().
• Documents why the project currently utilizes prisma db push instead of migrations, outlining the architectural trade-offs and future transition path to prisma migrate in docs/database-schema.md.
• Added unit tests in src/__tests__/schemaVerification.test.ts.