feat(taste): device-class context conditioning — #1551

Milestone #160 Opt 3b. Adds device class as a third context axis on top
of the #1531 time-of-day/weekday affinity: on the radio path, a candidate
is boosted when its artist concentrates in the current (daypart × weekday
× device) cell. Client-sent (client_id is opaque; no UA stored), so it's
captured going forward and applies to radio only (daily mixes are
cron-built with no device → stay device-agnostic).

Server:
- Migration 0048: play_events.device_class text NULL (no CHECK; normalized
  in Go — one whitelist entry per new client class, not a migration).
- events.go: eventRequest.device_class + normalizeDeviceClass (whitelist →
  mobile/web/…, else "other", empty → NULL); threaded through both
  RecordPlayStartedWithSource and RecordOfflinePlay into InsertPlayEvent.
- ListArtistContextPlayCountsForUser gains a current-device param; the cell
  FILTER adds AND ($2='' OR device_class=$2) — '' reproduces the #1531
  time-only behaviour exactly (used by mixes). SessionVector.DeviceClass
  carries it; the radio handler derives the current device from the user's
  latest play (GetLatestPlayDeviceClassForUser) — request-free proxy.
- No new tuning knob: device narrows the existing ContextAffinityScore
  (reuses context_time_weight).

Clients:
- web: play_started sends device_class 'web'.
- android: play_started + offline replay send 'mobile' (EventsWire +
  PlayOfflinePayload + MutationReplayer + PlayEventsReporter).

Test: LoadContextAffinity device-narrowing integration test (mobile vs web
artist separation; device-agnostic parity).

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

internal/db/queries/recommendation.sql

@@ -195,12 +195,14 @@ GROUP BY t.id, t.title, t.album_id, t.artist_id, t.duration_ms, t.file_path,
 
 -- name: ListArtistContextPlayCountsForUser :many
 -- Per-artist completed-play counts split by whether each play falls in the
--- CURRENT daypart × weekday-type cell, in the user's local timezone (#1531).
--- Feeds the context-affinity scoring term: an artist whose plays concentrate
--- in the current cell (vs the user's overall baseline, computed Go-side from
--- these rows) gets boosted right now. Skips excluded; a 365-day window bounds
--- cost. Daypart buckets: night [22,5) morning [5,12) afternoon [12,17)
--- evening [17,22). Weekend = ISO days 6–7 (Sat/Sun).
+-- CURRENT context cell, in the user's local timezone (#1531). The cell is
+-- daypart × weekday-type, optionally narrowed by device class (#1551): when
+-- $2 (the current device) is '', the device dimension is ignored (identical to
+-- the time-only #1531 behaviour, used by the daily mixes which have no device);
+-- when $2 is set (radio), a play only counts in the cell if its device_class
+-- matches. Feeds the context-affinity scoring term. Skips excluded; a 365-day
+-- window bounds cost. Daypart buckets: night [22,5) morning [5,12)
+-- afternoon [12,17) evening [17,22). Weekend = ISO days 6–7 (Sat/Sun).
 WITH tz AS (
     SELECT COALESCE(NULLIF(u.timezone, ''), 'UTC') AS zone
     FROM users u WHERE u.id = $1
@@ -219,6 +221,7 @@ now_cell AS (
 ),
 plays AS (
     SELECT t.artist_id,
+           pe.device_class,
            CASE
                WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 5  THEN 3
                WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 12 THEN 0
@@ -239,10 +242,21 @@ SELECT p.artist_id,
        count(*) FILTER (
            WHERE p.daypart = (SELECT daypart FROM now_cell)
              AND p.is_weekend = (SELECT is_weekend FROM now_cell)
+             AND ($2::text = '' OR p.device_class = $2::text)
        ) AS cell_plays
 FROM plays p
 GROUP BY p.artist_id;
 
+-- name: GetLatestPlayDeviceClassForUser :one
+-- The device_class of the user's most recent play (#1551), used as the
+-- "current device" for radio context conditioning. NULL when the latest play
+-- predates device capture or came from a client that didn't send one.
+SELECT device_class
+FROM play_events
+WHERE user_id = $1
+ORDER BY started_at DESC
+LIMIT 1;
+
 -- name: SuggestArtistsForUser :many
 -- M5c: per-user artist suggestions ranked by signal x similarity. The
 -- seeds CTE collects the user's likes (x5) plus recency-decayed plays