Add typed error classes for database failures

[deepa0128]

What

Introduces a typed exception hierarchy so callers can handle database errors by class rather than by parsing raw driver error objects.

DbError (base)
├── ConnectionError     — network / auth failures
├── ConstraintError     — unique, FK, not-null violations (carries .constraint name)
└── QueryTimeoutError   — statement killed after timeout

Each driver catches its native errors (pg SQLSTATE codes, mysql2 error codes) and maps them before they leave the driver boundary. All four error classes are exported from the package root.

Also included

• Postgres: adds pool.on('error') handler so idle-client failures don't crash the process
• MySQL: fixes SSL config ({} → { rejectUnauthorized: true }) to match Postgres behaviour
• Both drivers: refactors transaction handling with a makeTxDriver helper to reduce duplication