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/api/api.go

@@ -135,6 +135,12 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			// load failures). Admin-only inbox; any user can report.
 			authed.Post("/playback-errors", h.handleReportPlaybackError)
 
+			// Device diagnostics ingest (M9). Any signed-in user can
+			// POST a batch, but events are only stored when the
+			// account's debug_mode_enabled flag is on (handler no-ops
+			// otherwise). Admin views live under /admin/diagnostics.
+			authed.Post("/diagnostics", h.handleReportDiagnostics)
+
 			// Self-hosted in-app update channel (#397). Auth-gated to
 			// prevent anonymous bandwidth abuse on the APK stream;
 			// /apk additionally per-user rate-limited.
@@ -181,6 +187,11 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Delete("/users/{id}", h.handleAdminDeleteUser)
 				admin.Post("/users/{id}/reset-password", h.handleAdminResetPassword)
 				admin.Put("/users/{id}/auto-approve", h.handleAdminAutoApproveToggle)
+				admin.Put("/users/{id}/debug-mode", h.handleAdminDebugModeToggle)
+
+				// Device diagnostics timeline + device overview (M9).
+				admin.Get("/diagnostics", h.handleListAdminDiagnostics)
+				admin.Get("/diagnostics/devices", h.handleListAdminDiagnosticDevices)
 
 				admin.Get("/cover-sources", h.handleListCoverSources)
 				admin.Patch("/cover-sources/{provider_id}", h.handleUpdateCoverSource)