Feature — Optional Postgres Support

[coderkrp]

As a sysadmin,
I want Postgres support,
So that I can scale Ordo for larger workloads.

Acceptance criteria

• AC1: The application can connect to a PostgreSQL database when a DATABASE_URL for Postgres is provided.
• AC2: All repository implementations (for sessions, idempotency, etc.) work correctly with the Postgres backend.
• AC3: The application continues to work with SQLite when a SQLite DATABASE_URL is provided.
• AC4: The README is updated with instructions on how to configure the service for Postgres.

Definition of Done

• Code implemented in a feature/fix branch
• Unit / integration tests added or existing tests updated
• Linting passed and code builds locally
• Docs / README updated if behavior changed
• PR opened and linked (Closes #<issue-number>)
• Manual QA steps verified (if applicable)