Add playlists, credentials, and jobs pages; wire up dashboard

Vendored htmx.min.js (1.9.12) and alpine.min.js (3.14.1) into static/ --
self-hosted per the no-CDN/CSP requirement, included in base.html.

routers/playlists.py: list/add/remove playlists, a detail page showing
live per-playlist status via status_service (Spotify vs. beets vs. sldl
index vs. quarantine), edit form for active/cron_expr/no_m3u/notes, and a
"Run now" button wired to the jobs run-now endpoint.

routers/credentials.py: write-only credential forms per scope (a saved
secret is never sent back to the browser -- fields just show "(set)" and
leaving a field blank keeps its current value, matching
credential_service.set_credentials' partial-update semantics).

routers/jobs.py: maintenance job list with run-now/enable-disable, and a
recent-runs table that auto-refreshes via HTMX polling
(hx-trigger="every 5s") against a partial-only endpoint -- the one place
in the UI where avoiding a full-page reload actually matters, since job
status changes while the page is sitting open. Per-run log viewing.

Dashboard now shows real data (beets stats, playlist counts, recent job
runs) instead of the placeholder from Task 2.

Verified all 10 authenticated pages render successfully (200, no template
errors) via TestClient with require_auth overridden, including the
playlist detail page's graceful-degradation path when Spotify credentials
aren't configured yet (renders a "couldn't fetch live status" message
instead of a 500).

app/routers/dashboard.py

@@ -1,16 +1,31 @@
 from fastapi import APIRouter, Depends, Request
 from fastapi.templating import Jinja2Templates
+from sqlalchemy import select
 
+from app.db import get_db
+from app.models import JobRun
 from app.security.deps import require_auth
+from app.services import beets_service, playlist_service
 
 router = APIRouter(tags=["dashboard"])
 templates = Jinja2Templates(directory="app/templates")
 
 
 @router.get("/")
-async def dashboard(request: Request, user: dict = Depends(require_auth)):
+async def dashboard(request: Request, user: dict = Depends(require_auth), db=Depends(get_db)):
+    stats = beets_service.stats()
+    playlists = playlist_service.list_all(db)
+    recent_runs = list(
+        db.execute(select(JobRun).order_by(JobRun.started_at.desc()).limit(10)).scalars()
+    )
     return templates.TemplateResponse(
         request,
         "dashboard.html",
-        {"user": user},
+        {
+            "user": user,
+            "stats": stats,
+            "playlist_count": len(playlists),
+            "active_playlist_count": len([p for p in playlists if p.active]),
+            "recent_runs": recent_runs,
+        },
     )