[FEATURE] Migrate SQLite schema to support PostgreSQL as optional backend

[ritik4ever]

Category: backend / database
Project area: backend/src/services/db.ts

Context:
ADR 0001 documents the SQLite choice and migration path to PostgreSQL.

Implementation:

• Abstract DB layer behind repository interface.
• SQLite remains default; PostgreSQL activated via DATABASE_URL env var.
• Migration scripts compatible with both.

Acceptance Criteria:

• DATABASE_URL switches backend to PostgreSQL.
• All tests pass with both drivers.
• ADR updated with implementation notes.