The inline dict cache was a latent bug: no lock (the engine singleton is hit by
concurrent Flask + worker threads) and a wholesale-clear cliff at 256 (nuked hot
entries). Extracted a thread-safe TTL+LRU TTLCache into an importable core/ module
with seam tests (expiry via injected clock, LRU-not-wholesale eviction, a
concurrency stress test). Engine now uses it; search is cached too (60s, ownership
re-stamped fresh). Deliberately NOT persisted to disk — durable data already lives
in video.db; that tier would be over-engineering for a self-hosted app.
