Replaces the M2 stub /api/radio with real weighted-shuffle scoring:
LikeBoost + recency_decay - skip_ratio penalty + jitter, hard
suppression of last-hour plays. Wide candidate pool (whole library)
for v1; M4 adds similarity-based pool refinement. Pure scoring
function in internal/recommendation; live-DB candidate loader; HTTP
handler is a thin shim. No web changes — existing playRadio action
already calls /api/radio.
Sub-plan #1 of 3 in M3 (Fable #340). Session vectors and contextual
match score land in the next two sub-plans.
