fix n+1 queries in comfortGenerator and deepDiveGenerator (#110)

both generators ran N sequential queries per artist/album.
rewritten with ROW_NUMBER() OVER (PARTITION BY ...) to get per-group
limits in a single round-trip, preserving existing semantics:
- comfortGenerator: up to 2 tracks per artist (was 20 queries → 1)
- deepDiveGenerator: up to 5 tracks per album (was 20 queries → 1)

PLANS.md

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+# muzick — overnight fix plan
+
+## tasks
+1. **fix N+1 queries** in generators.service.ts (#110, prio:4)
+2. **fix SSRF guard** in images.routes.ts (#111, prio:3)
+3. **clean & re-enrich images** (#124, prio:0 — operational)
+4. **ethos UI migration** (#34, prio:5 — dispatched to ethos-ui agent)
+
+## 1. N+1 queries
+
+### comfortGenerator (lines 57-87)
+current: loops 20 artists, `SELECT ... WHERE object_id = $1` per artist
+fix: single query with `WHERE cf.object_id = ANY($1::uuid[])` + `unnest` to get per-artist rows back
+
+### deepDiveGenerator (lines 248-274)
+current: loops 20 albums, `SELECT ... WHERE t.album_id = $1` per album
+fix: single query with `WHERE t.album_id = ANY($1::uuid[])` — already batched on artist query
+
+### verification
+`comfortGenerator` and `deepDiveGenerator` are tested in `generators.test.ts`