fix: slim Docker Compose for dev and restore Prisma/backend imports

[king-aj-the-first]

closes #947
closes #948
closes #951
closes #952

PR description

## Summary
- Add Docker Compose overrides so local development starts only PostgreSQL + standalone Redis, while keeping the full Sentinel/Cluster stack available for production-like and HA testing.
- Confirm Prisma schema is present and generates successfully; confirm broken Prisma client and auth middleware imports in the affected backend services are resolved.

## Changes

### Task 1 — Docker Compose (dev vs full vs HA)
- **`docker-compose.dev.yml`**: override that disables read-replica, Sentinels, Cluster nodes, and backend via Compose profiles so only `db` + `redis` start.
- **`docker-compose.prod.yml`**: HA override that wires the backend to Sentinel/Cluster (`REDIS_MODE` default `sentinel`) and adds healthcheck dependencies on Sentinel/Cluster services.
- **`docker-compose.yml`**: unchanged as the full production-like stack.
- **Docs**: README, Redis setup/caching guides, and architecture/general READMEs updated with the recommended workflow:
  `docker compose -f docker-compose.yml -f docker-compose.dev.yml up`

### Task 2 — Prisma schema
- `backend/prisma/schema.prisma` already defines models used by the seed script and services (`Student`, `Course`, `Enrollment`, `Idea`, `Vote`, `HashSimulation`, `P2PNode`, `P2PMessage`, etc.).
- Verified: `npx prisma validate` and `npx prisma generate` succeed.
- `npx prisma db push` requires a running Postgres (start via the new dev compose), then:
  ```bash
  cd backend && npx prisma db push

Task 3 — Prisma imports

• Fixed paths in:
  • backend/src/dashboard/hash.service.ts
  • backend/src/infrastructure/p2p.service.ts
  • backend/src/simulator/voting.service.ts
• All import from ../db/index.js (Prisma client singleton). No remaining ../db/prisma.js references under backend/src.

Task 4 — Auth middleware import

• backend/src/infrastructure/infrastructure.routes.ts imports authenticate from ../auth/auth.middleware.js (correct module; ../middleware/auth.ts exports a different helper and is not the app-wide auth middleware).
• tsc no longer reports TS2307 for this import.

Test plan

• docker compose -f docker-compose.yml -f docker-compose.dev.yml config --services → only db and redis
• docker compose -f docker-compose.yml -f docker-compose.dev.yml up -d starts Postgres + Redis without Sentinels/Cluster
• docker compose -f docker-compose.yml up -d still brings up the full stack
• docker compose -f docker-compose.yml -f docker-compose.prod.yml up -d starts HA-oriented stack
• cd backend && npx prisma generate succeeds
• cd backend && npx prisma db push creates tables against local Postgres
• npx tsc --noEmit has no TS2307 for db/prisma or auth middleware on the four task files

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[ayomideadeniran]

pr under review