feat(cache): add debug cache diagnostics

[n30nex]

What this PR does

Add debug records for cache hits, misses, invalid entries and Redis read-error bypasses. Each record identifies the cache component without including a cache key, value or raw Redis error. Fetch and fallback behavior stays unchanged, and info-level operation remains quiet.

Related to #51. This is independent of logging-controls PR #144: it uses the standard slog default logger and becomes visible when a debug handler is configured.

Type of change

• New feature
• Tests

Checklist

• go build ./... passes
• gofmt -l . is empty
• go vet ./... passes
• go test ./... passes
• New behavior has tests
• No DB, API or dependency changes
• I have read CONTRIBUTING.md

Testing notes

Native Pi 5 build, vet and full tests pass with PostgreSQL tests enabled. The cache suite and focused Windows race check pass. A miniredis regression verifies miss/hit/corrupt-entry records, fetch/result behavior, absence of synthetic private keys/values and silence at info level.

CI and CodeQL pass. The combined Pi preview includes this change with #144; JSON/info operation is quiet for cache diagnostics, both feeds advance and public API/WebSocket checks pass. Debug behavior is verified by the native cache tests.

AI-assisted implementation and testing under the contributor's standing authorization; submitted for maintainer review.

[n30nex]

@MrAlders0n please review this change with your Claude agent as part of the September 13 Beacon review batch. The current candidate is 85f861ccf0cb; validation and Pi evidence are in the PR description.

Please check cache hit/miss/error diagnostics without private keys/values or behavior changes. Issue #51 requires both #144 and this PR. Preferred sequence: merge this cache slice first, then complete the logging controls in #144 and add the closing reference there. The issue remains open while either required part is missing.