Backend: Add PostgreSQL support as alternative to SQLite

[joelpeace48-cell]

Summary

The backend uses SQLite via the DAL abstraction (backend/src/dal/). SQLite is great for development and single-instance setups but is unsuitable for production deployments that require horizontal scaling, concurrent writes at volume, or managed cloud database hosting. The DAL abstraction already separates repository interfaces from implementations — adding a PostgreSQL backend is the natural next step.

Problem

• SQLite has limited concurrent write throughput (WAL mode helps but doesn't eliminate the bottleneck)
• No native replication or failover
• Incompatible with multi-instance (horizontal) deployments
• Cloud platforms (Railway, Render, Fly.io) recommend PostgreSQL for persistent workloads

Acceptance Criteria

• Create backend/src/dal/pgCampaignRepository.js and backend/src/dal/pgAuditLogRepository.js implementing the same interfaces as the SQLite versions
• Use pg (node-postgres) or @databases/pg as the database client
• Add a DATABASE_URL env variable; when present and prefixed with postgres://, use the PG implementation
• Add SQL migration files in backend/src/db/migrations/ compatible with both SQLite and PostgreSQL schemas
• Add integration tests using a real PostgreSQL instance (can use Docker in CI)
• Update backend/.env.example with PostgreSQL configuration examples
• Update compose.yaml to add an optional postgres service profile (like the existing redis profile)
• Document PostgreSQL setup in backend/README.md

References

• backend/src/dal/index.js — DAL factory
• backend/src/dal/sqliteCampaignRepository.js — reference implementation
• compose.yaml — Docker Compose for adding postgres service
• backend/.env.example

[ShantelPeters]

@ShantelPeters has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

please can i be assigned to this issue , i am ready to work please

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[pre-cious-Igwealor]

@pre-cious-Igwealor has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

can i work on this issue?

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