feat(taste): phase 2b — taste_overlap candidate arm (#796)

2a re-ranks the existing pool by TasteMatch; this ensures taste-relevant tracks
ARE in the pool. Adds a 6th arm to LoadRadioCandidatesV2: in-library tracks by
the user's top positively-weighted taste-profile artists ($10 K, weight > 0,
deterministic weight-DESC,id order so it doesn't reintroduce same-day
nondeterminism). Pool-inclusion only (sim_score 0) — TasteMatch already scores
the fit. Empty for cold-start users (no profile).

- CandidateSourceLimits.TasteOverlap; default 20 (radio), 80 for For-You via
  systemForYouSourceLimits.
- You-might-like deliberately sets TasteOverlap=0: it surfaces NOT-actively-
  engaged artists, so flooding its pool with top-taste (mostly already-played)
  artists would just feed the read-time dedup.
- Test: positive-weight artist's track enters via the arm; negative-weight one
  is excluded (weight > 0). Existing pool tests unaffected (no profile seeded).

Deferred within 2b: profile-seeded For-You — marginal given the arm + TasteMatch
already inject taste broadly (top-played seed ≈ top-taste artist).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/recommendation/candidates.go

@@ -72,6 +72,10 @@ type CandidateSourceLimits struct {
 	TagOverlap    int
 	LikesOverlap  int
 	RandomFill    int
+	// TasteOverlap (#796 phase 2b): tracks by the user's top positively-
+	// weighted taste-profile artists. 0 disables the arm (e.g. cold-start
+	// users have an empty profile, so it contributes nothing anyway).
+	TasteOverlap int
 }
 
 // DefaultCandidateSourceLimits returns the v1 hardcoded constants per spec.
@@ -82,6 +86,7 @@ func DefaultCandidateSourceLimits() CandidateSourceLimits {
 		TagOverlap:    20,
 		LikesOverlap:  20,
 		RandomFill:    30,
+		TasteOverlap:  20,
 	}
 }
 
@@ -113,6 +118,7 @@ func LoadCandidatesFromSimilarity(
 		Limit_3: int32(limits.TagOverlap),
 		Limit_4: int32(limits.LikesOverlap),
 		Limit_5: int32(limits.RandomFill),
+		Limit_6: int32(limits.TasteOverlap),
 	})
 	if err != nil {
 		return nil, err