feat: add read-replica routing and query load-balancing

[mallison031]

Closes #123

Adds a routing layer to src/db.ts that automatically routes read-heavy

traffic to replicas while sending all writes to the primary database,
improving throughput and isolating ingest contention.

### What Changed
- Created `src/db/router.ts` with a `withReadReplicas` Prisma Client

Extension.
- Reads (findMany, findUnique, count, etc.) and raw queries
($queryRaw) are now round-robined across DATABASE_REPLICAS.
- Updated src/db.ts to wrap the exported prisma singleton in the
router.
- Added comprehensive unit tests for the router in
src/db/__tests__/router.test.ts.

### Key Design Decisions
- **Read-Your-Writes:** Leveraged Prisma extensions' native context to

check if a query is executing inside a $transaction. Interactive
transactions bypass replicas and route strictly to the primary, cleanly
preserving read-your-writes guarantees for all grouped operations in the
repo.
- Health Checks & Failover: A background loop continuously pings
replicas via SELECT 1. If a replica fails a ping—or if a query fails mid-
flight—it's marked unhealthy, temporarily removed from the round-robin, and
traffic gracefully falls back to the primary.

### Acceptance Criteria
- [x] Reads load-balanced across healthy replicas
- [x] Unhealthy replica removed; traffic fails over to primary
- [x] Tests for routing and failover
- [x] Respects read-your-writes (automatically routes all `$transaction`

queries to primary)

### Test & Coverage
- Tests pass locally.
- Router logic achieves `100%` function and `97%` line coverage.

### Security Note
Replicas are passed via the standard environment configuration

(DATABASE_REPLICAS). Connection logic avoids exposing URI components or
logging credentials.

Closes #118

[Miracle656]

Solid implementation of read-replica routing (#118). withReadReplicas uses a Prisma $extends query interceptor to send reads to round-robined healthy replicas while writes and anything inside a transaction go to the primary; the background health-check loop marks replicas unhealthy and fails over to primary, and interval.unref() keeps the process exitable. It's wired in cleanly via DATABASE_REPLICAS with a zero-config passthrough default (no replicas → primary unchanged), and the tests cover read/write routing, transaction bypass, round-robin, and unhealthy fallback. Merging — nice work!

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