feat(server/diagnostics): device debug-reporting ingest + admin timeline + retention (M9)

New diagnostic_events table + per-account users.debug_mode_enabled flag.
When an account's flag is on, its client(s) POST a batch timeseries of
connectivity / UPnP-sync / power / lifecycle events to /api/diagnostics
(no-op 204 when off, kind whitelist mirrors the CHECK constraint).

Admin surface: GET /api/admin/diagnostics (optional account/device/kind/
time-window filters, RFC3339-or-epoch-ms, export-sized paging) + a
/diagnostics/devices overview + PUT /api/admin/users/{id}/debug-mode to
flip an account remotely while a bug is live. debug_mode_enabled is now
exposed on /api/me (client gate) and the admin user views.

Retention: a 30-day gc-worker sweep (GcPruneDiagnostics), keyed on the
server clock so a skewed device clock can't keep rows alive.

Refs Scribe M9 (#119), tasks #1172 #1173.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K55iTxn95BtshocgdE1shW

internal/db/dbq/models.go

@@ -265,6 +265,18 @@ type CoverArtSourcesMetum struct {
 	LastRegisteredProvidersHash string
 }
 
+type DiagnosticEvent struct {
+	ID         pgtype.UUID
+	UserID     pgtype.UUID
+	ClientID   string
+	AppVersion *string
+	OsVersion  *string
+	Kind       string
+	Payload    []byte
+	OccurredAt pgtype.Timestamptz
+	ReceivedAt pgtype.Timestamptz
+}
+
 type GeneralLike struct {
 	UserID  pgtype.UUID
 	TrackID pgtype.UUID
@@ -552,6 +564,7 @@ type User struct {
 	Email               *string
 	Timezone            string
 	TimezoneUpdatedAt   pgtype.Timestamptz
+	DebugModeEnabled    bool
 }
 
 type UserInvite struct {