perf: pre-compute ETag for /all/ endpoints

[deanmoses]

Summary

• Pre-serialize JSON and pre-compute ETag at cache-build time instead of storing raw Python dicts. On cache hit, return a pre-built HttpResponse with the ETag already set so ConditionalGetMiddleware can return 304 without any JSON serialization or MD5 hashing.
• Drops 304 latency from ~669ms to ~1ms for /api/titles/all/ (and equivalently for /api/models/all/, /api/manufacturers/all/, /api/people/all/).
• 200 cache-hit responses also become ~1ms since the JSON is pre-serialized.
• Gracefully handles stale cache entries from the old list[dict] format (treats them as a cache miss and rebuilds).

Test plan

• All 1473 existing backend tests pass
• All 366 frontend tests pass
• New TestConditionalGet tests verify 304 on matching ETag, 200 on stale ETag, and cache format
• Manual: curl -v /api/titles/all/ returns ETag header
• Manual: curl -v -H 'If-None-Match: <etag>' /api/titles/all/ returns 304 in sub-10ms

🤖 Generated with Claude Code

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