feat(db): add contextual_likes table (M3 session-context capture)

Table was referenced in migration 0005's comment but never created —
this slice fills the gap. Soft-delete via deleted_at column; hot-path
partial index on active rows; GIN index on session_vector for
M3 sub-plan #3's similarity queries.

internal/db/migrations/0007_contextual_likes.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS contextual_likes;

internal/db/migrations/0007_contextual_likes.up.sql

@@ -0,0 +1,32 @@
+-- contextual_likes captures a per-session-context snapshot at the time of
+-- each like. The recommendation engine in M3 sub-plan #3 uses these rows
+-- to compute contextual_match_score per (current session, candidate track).
+--
+-- Multiple rows per (user, track) are allowed and expected — each row
+-- represents a like in a specific session context. Soft-deleted rows
+-- remain in the table (deleted_at populated) but are filtered out of the
+-- engine's queries via the partial index. Re-liking a previously-unliked
+-- track adds a NEW row (does not undelete the old one).
+--
+-- Note: the M2 events migration (0005) commented that contextual_likes
+-- "ships nullable now" but the actual CREATE TABLE was missed. This slice
+-- ships the table for the first time.
+
+CREATE TABLE contextual_likes (
+    id              uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id         uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    track_id        uuid        NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    liked_at        timestamptz NOT NULL DEFAULT now(),
+    deleted_at      timestamptz,
+    session_vector  jsonb,
+    session_id      uuid REFERENCES play_sessions(id) ON DELETE SET NULL
+);
+
+-- Hot path for both the engine's lookups and the soft-delete UPDATE.
+CREATE INDEX contextual_likes_active_idx
+    ON contextual_likes (user_id, track_id)
+    WHERE deleted_at IS NULL;
+
+-- Vector similarity queries from M3 sub-plan #3 will use this.
+CREATE INDEX contextual_likes_vector_idx
+    ON contextual_likes USING gin (session_vector);