feat: add /health endpoint and HTTP server

[Lewiskunta]

Implements: /health endpoint and HTTP server startup.

Changes:

• Installed github.com/go-chi/chi/v5 and github.com/jackc/pgx/v5
• Created internal/db/db.go - connection pool helper using pgxpool
• Created internal/api/router.go - Chi router with Logger and
  Recoverer middleware
• Created internal/api/health.go - GET /health handler
  • Returns HTTP 200 with {"status":"ok","database":"ok",...}
  • Returns HTTP 503 with {"status":"degraded","database":"unreachable",...}
  • 3-second database ping timeout prevents hanging health checks
• Replaced cmd/server/main.go placeholder with real HTTP server
  • Configurable read/write/idle timeouts
  • Graceful shutdown on SIGINT/SIGTERM with 30-second drain period
• Tests for health response structure and statusFromHTTP helper

Verified manually:

• GET /health returns 200 when postgres is running
• GET /health returns 503 when postgres is stopped
• Server shuts down cleanly on Ctrl+C