Prevent stale tracker responses after deletion

[ALeonard9]

Summary

• mark viewer- and tracker-specific reads as private, no-store
• cover public profiles, user rankings, global search, and per-domain catalog search through one central policy
• prevent browsers, CDNs, and framework data caches from retaining mutable responses under URL-only keys
• add deletion consistency coverage for movies, TV, books, and games

Architecture note

The API has no Redis/profile cache or search index to evict: these endpoints query live SQL. The stale-data risk was HTTP response caching, so this removes the cache layer instead of introducing invalidation key bookkeeping.

Validation

• 785 tests passed
• black passed
• pylint passed at 10.00/10
• commit and changed-test pre-push hooks passed
• live dev API returned Cache-Control: private, no-store for /v1/public/*
• regression tests verify rank -> public/rankings GET -> delete -> immediate GET for all four domains

Closes #297