docs: fix stale-live docs and drop obsolete Obsidian spec

Refreshes the docs that were actively misleading about the current
FastAPI + React + service-registry app, and deletes one obsolete design.

- CONTRIBUTING.md: full rewrite — Streamlit-era guidance replaced with
  the current backend (ruff/pytest, src/ layout) + frontend (npm
  lint/build/test) workflow, service-registry model, and shadcn/Tailwind
  stack. Mirrors AGENTS.md.
- README.md: removed the non-existent /addons/:addonId route (Services
  page is current); fixed the per-machine Jellyfin wording; replaced the
  py_compile dev snippet with ruff + pytest / npm lint+build+test.
- backend/README.md: updated the structure tree (removed deleted
  clients/resources.py; added routers backups/services/tasks/widgets,
  integrations/, models/, widgets/, workers/); dropped the "starts the
  collector" sentence (MonitoringPoller is decommissioned).
- frontend/README.md: corrected the uvicorn module path
  (main:app -> media_library_viewer_api.main:app).
- Deleted docs/superpowers/specs/2026-05-08-obsidian-documentation-design.md
  (Obsidian vault never built; stack refs MUI/D3/AG Grid all removed).

Historical docs (MIGRATION_PLAN, superpowers backup-monitoring, the
bannered design/runbook/context files) deferred to a later banner pass.

frontend/src/pages/Settings.tsx

@@ -1077,7 +1077,10 @@ export function Settings() {
 													</>
 												) : (
 													<FormField label="Local hint">
-														<Input value="Uses the API host directly" disabled />
+														<Input
+															value="Uses the API host directly"
+															disabled
+														/>
 													</FormField>
 												)}
 											</div>