Unify db connections, add otel to pool

[djeebus]

1. The pgxpool.Pool is the only way of connection to the database. Creation is centralized in db.NewPool
2. If a *sql.DB is needed, db.Open(pool) can provide one. Connection pooling is still handled by the pgxpool.Pool.

---

Note

Centralizes Postgres connections via a shared pgxpool with OpenTelemetry, injects *sql.DB/pgxpool into services, and updates API, seeds, and tests to use the new pool and migration check.

• Database (shared pkg):
  • Introduce db.NewPool (pgxpool) and db.Open to obtain *sql.DB; enable OTEL via otelpgx (traces + metrics).
  • Change db.NewClient to accept *sql.DB instead of creating connections.
• API:
  • Config: add POSTGRES_MAX_CONNECTIONS and POSTGRES_MIN_IDLE_CONNECTIONS.
  • Main: create pool via db.NewPool, open *sql.DB, pass into handlers.NewAPIStore.
  • utils.CheckMigrationVersion now takes *sql.DB (no internal open/close).
  • handlers.NewAPIStore signature updated to receive injected DB (*sql.DB, *pgxpool.Pool).
• SQLC client (packages/db/client):
  • Simplify NewClient to NewClient(pool *pgxpool.Pool); remove internal pool/options and tx helpers.
• Docker Reverse Proxy:
  • Build pool in main, open *sql.DB, pass to handlers.NewStore; use shared db.NewClient with provided conn.
  • Add cache.ErrNotInCache sentinel error.
• Local Dev + Seeds + Tests:
  • Migrate to db.NewPool/db.Open; update seed scripts and test helpers to use injected pool/clients.
• Dependencies:
  • Add github.com/exaring/otelpgx and related OTEL/pgx deps across modules.

^{Written by Cursor Bugbot for commit 2109e4d. This will update automatically on new commits. Configure here.}