Feature/tests and quality gates

[Howard-nolan]

Change Summary:

• Expanded coverage with table-driven handler unit tests (sqlmock) and an integration-tagged black-box test that exercises real HTTP on an in-memory DB.
• Wired server to new middleware/logger/cache-aware handler signature and recorded supporting dependencies (prometheus, hashicorp/golang-lru, sqlmock).

How It Works:

• Unit tests in internal/http/handlers/handlers_test.go drive handlers through table cases with sqlmock; integration_test.go (build tag integration) spins up httptest.NewServer with in-memory SQLite to hit /v1/shorten then /v1/r/{code} end-to-end.

Additional Notes (optional):

• Run integration suite with go test -tags=integration ./...; it’s skipped in default runs.