fix(api): clamp for you recency decay

[raymondjacobson]

Summary

• clamps the For You recency-decay exponent before calling exp(...)
• keeps very old candidates ranked as effectively zero-recency instead of returning a 500
• adds a regression test with a year-1900 trending candidate

Production evidence

• during the perf(db): index for you trending tie order #905/perf(db): index playlist trending order #906 release watch, /v1/users/:userId/feed/for-you returned 500s with SQLSTATE 22003 underflow for several users
• a read-only prod probe reproduced the raw failure with exp((-1000000)::double precision)
• the exact clamped expression against a year-1900 timestamp returned a finite 9.85967654375977e-305 score

Verification

• go test ./api -run 'TestV1FeedForYou' -count=1 -timeout=3m\n- git diff --check