feat(db): add M2 schema — play_sessions, play_events, skip_events

Tables and indexes per spec §5. session_vector_at_play ships nullable
so M3 doesn't need a follow-up migration. Table is named play_sessions
to avoid collision with the existing sessions (HTTP auth) table from
migration 0004.

internal/db/migrations/0005_events.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS skip_events;
+DROP TABLE IF EXISTS play_events;
+DROP TABLE IF EXISTS play_sessions;

internal/db/migrations/0005_events.up.sql

@@ -0,0 +1,46 @@
+-- M2 listening telemetry: play_sessions, play_events, skip_events.
+-- Note the table is named play_sessions (not sessions — that table already
+-- exists from migration 0004 and holds HTTP auth sessions). Listening
+-- sessions are an entirely separate concept driven by spec §6.
+--
+-- session_vector_at_play and contextual_likes ship as nullable now even
+-- though M2 writes NULL; M3 will populate them without a follow-up
+-- migration.
+
+CREATE TABLE play_sessions (
+    id              uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id         uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    started_at      timestamptz NOT NULL,
+    ended_at        timestamptz,
+    last_event_at   timestamptz NOT NULL,
+    track_count     integer     NOT NULL DEFAULT 0,
+    client_id       text
+);
+CREATE INDEX play_sessions_user_last_event_idx ON play_sessions (user_id, last_event_at DESC);
+
+CREATE TABLE play_events (
+    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,
+    session_id               uuid             NOT NULL REFERENCES play_sessions(id) ON DELETE CASCADE,
+    started_at               timestamptz      NOT NULL,
+    ended_at                 timestamptz,
+    duration_played_ms       integer,
+    completion_ratio         double precision,
+    was_skipped              boolean          NOT NULL DEFAULT false,
+    client_id                text,
+    session_vector_at_play   jsonb,
+    scrobbled_at             timestamptz
+);
+CREATE INDEX play_events_user_started_idx ON play_events (user_id, started_at DESC);
+CREATE INDEX play_events_user_track_idx   ON play_events (user_id, track_id);
+
+CREATE TABLE skip_events (
+    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,
+    session_id   uuid        NOT NULL REFERENCES play_sessions(id) ON DELETE CASCADE,
+    skipped_at   timestamptz NOT NULL,
+    position_ms  integer     NOT NULL
+);
+CREATE INDEX skip_events_user_skipped_idx ON skip_events (user_id, skipped_at DESC);