fix(recommendation): don't shrink a candidate arm ordered by unseeded random()

Fixes the integration failure from f367eeaa:
"same-day rebuild produced different track lists".

Cutting RandomFill 30→10 for Songs-like broke
TestBuildSystemPlaylists_DailyNonceDeterminism, and the reason is worth
stating because the number is not the bug.

`likes_overlap` and `random_fill` end in a bare `ORDER BY random()` with no
daily seed (recommendation.sql:118, :161). Such an arm returns a STABLE set
only while its LIMIT exceeds the rows eligible for it — then it returns all
of them, and the random order stops mattering because scoreAndSortCandidates
sorts by track id before drawing jitter. Below that threshold the arm
returns a random SUBSET, and two builds on the same day draw different ones.

So the test was green by accident. It seeds ~20 tracks against a default
RandomFill of 30; the limit exceeded the library, so the arm returned
everything. Determinism held for a reason unrelated to the code being right.

Which means it does NOT hold in production. Any real library is larger than
30, so same-day rebuilds have been drawing different mixes since that arm
was written — invisible, because a mix changing after a refresh looks like a
feature. Filed as #3889; the fix is a seeded ordering per (user, day), which
needs a .sql change and sqlc regeneration and so cannot land from here.

The correction: grow an arm freely, never shrink one whose ordering is
unseeded random. LikesOverlap and RandomFill go back to the defaults;
TasteOverlap stays halved because it sorts by `tpa.weight DESC, t.id` and is
genuinely deterministic. Guarded by a test that names the reasoning, so the
next person to trim these has to read why first — and it should be DELETED
once #3889 lands rather than worked around.

The cost is honest: the seed-independent share of the Songs-like pool falls
from 29% to 20% instead of the intended cut. That matters less than it
sounds. The pool only biases the draw; the songs_like WEIGHTS are what
actually demote sim_score-0 candidates, and they are untouched here — a
perfect match still scores 5.00 against an unrelated favourite's 2.00.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

internal/recommendation/candidates.go

@@ -139,22 +139,46 @@ func DefaultCandidateSourceLimits() CandidateSourceLimits {
 // would produce a short mix or none at all, and "no playlist" is a worse
 // answer than "a few tracks further from the seed than we would like".
 //
-// likes_overlap is cut hardest of the tier-2 arms for a specific reason: its
-// SQL assigns a FLAT 0.6 sim_score (recommendation.sql:108) rather than
-// measuring anything. It is a collaborative signal wearing similarity's
-// clothes, and raising SimilarityWeight amplifies it — if real ListenBrainz
-// scores commonly land below 0.6 it would outrank genuine matches. Halved
-// here pending the fill-rate measurement in #3879; the honest fix is to stop
-// it claiming a similarity score it did not compute.
+// DO NOT SHRINK AN ARM ORDERED BY UNSEEDED random(). This is the constraint
+// that shapes the numbers below, and it is not obvious from reading them.
+//
+// `likes_overlap` and `random_fill` both end in a bare `ORDER BY random()`
+// (recommendation.sql:118, :161) with no daily seed. Such an arm returns a
+// STABLE set only while its LIMIT exceeds the rows eligible for it — at that
+// point it returns all of them and the random order is irrelevant, because
+// the caller sorts by id before scoring. Drop the limit below the eligible
+// count and the arm starts returning a random SUBSET, which differs between
+// two builds on the same day.
+//
+// That is a real defect (#3889) rather than a quirk of this function, and it
+// bit here: cutting RandomFill to 10 broke
+// TestBuildSystemPlaylists_DailyNonceDeterminism, whose library is smaller
+// than the default limit and whose determinism was therefore accidental.
+// Growing an arm is always safe; only shrinking one is.
+//
+// So the seed-independent arms are trimmed only where the ordering is
+// deterministic: `taste_overlap` sorts by `tpa.weight DESC, t.id` and can be
+// cut, `random_fill` cannot. The reduction is consequently modest — and it
+// matters less than it looks, because the WEIGHTS are what demote sim_score-0
+// candidates now. The pool change biases the draw; the songs_like profile is
+// what actually keeps unrelated tracks out of the result.
+//
+// One arm is left alone that arguably should not be: `likes_overlap` assigns
+// a FLAT 0.6 sim_score (recommendation.sql:108) rather than measuring
+// anything — a collaborative signal wearing similarity's clothes, which a
+// raised SimilarityWeight amplifies. If real ListenBrainz scores commonly
+// land below 0.6 it will outrank genuine matches. It cannot be trimmed here
+// without the determinism fix landing first; the honest repair is to stop it
+// claiming a similarity score it never computed (#3879).
 func SongsLikeCandidateSourceLimits() CandidateSourceLimits {
 	return CandidateSourceLimits{
 		LBSimilar:     60, // tier 1 — doubled; the only arm that measures the seed
-		SimilarArtist: 40, // tier 2
+		SimilarArtist: 40, // tier 2 — raised; growing is always safe
 		TagOverlap:    20, // tier 2
 		UserCoplay:    20, // tier 2
-		LikesOverlap:  10, // tier 2, halved — flat 0.6, see above
-		TasteOverlap:  10, // tier 3 floor — halved, not removed
-		RandomFill:    10, // tier 3 floor — cut hard, never to zero
+		LikesOverlap:  20, // tier 2 — NOT trimmed: unseeded random(), see above
+		TasteOverlap:  10, // tier 3 floor — halved; deterministic ordering, safe
+		RandomFill:    30, // tier 3 floor — NOT trimmed: unseeded random(), see above
 	}
 }