feat(admin): SFM + SLMM diagnostic pages under Developer settings

New /admin/sfm page (linked from Settings → Developer):
- Health banner — green/red with version + last-checked timestamp
- Connection panel — shows SFM_BASE_URL terra-view is configured with
- 4 KPI tiles — known units, total events, stale monitor_log rows,
  stale ach_sessions rows (the deprecated tables from the paused
  Python-ACH experiment, useful for confirming nothing's growing them)
- Per-unit roll-up table — serial, last_seen, event count, stale
  per-unit counts, sourced from SFM's /db/units
- Recent events with forwarding latency — color-coded gap between
  the event's recorded timestamp and SFM ingest time, so operators
  can spot watchers that are forwarding stale files (e.g. after a
  jobsite outage)
- Raw API tester — text input + GET button against any /api/sfm/*
  path, response rendered as prettified JSON

New /admin/slmm page — same layout, stripped down to health + connection
+ raw API tester.  For per-device SLM control the existing
/sound-level-meters dashboard remains the right entry point.

Backend (backend/routers/admin_modules.py):
- GET /admin/sfm, GET /admin/slmm — HTML pages
- GET /api/admin/sfm/overview — single aggregated probe that returns
  health, units, last 25 events with computed latency, stale-table
  counts, cache stats.  Tolerant of partial failures: any sub-fetch
  error is captured into errors{} so a flaky SFM endpoint doesn't
  break the whole page
- GET /api/admin/slmm/overview — health + connection info only for now

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/main.py

@@ -106,6 +106,9 @@ app.include_router(settings.router)
 from backend.routers import watcher_manager
 app.include_router(watcher_manager.router)
 
+from backend.routers import admin_modules
+app.include_router(admin_modules.router)
+
 # Projects system routers
 app.include_router(projects.router)
 app.include_router(project_locations.router)