Add FastAPI app skeleton: settings, db, models, security, auth

- settings.py: pydantic-settings config for the two mount points
  (MUSIC_DATA_DIR, ALEMBIC_CONFIG_DIR), OIDC client config, encryption key path
- db.py: SQLite engine with WAL mode, plain-SQL schema migration runner
  (schema/*.sql, tracked by schema_version -- not the alembic migration tool)
- models.py: SQLAlchemy ORM models matching schema/0001_init.sql
- security/crypto.py: Fernet encrypt/decrypt for the secrets table
- security/oidc.py + routers/auth.py: Pocket ID OIDC login/callback/logout
- security/deps.py: require_auth dependency (redirect-to-login on no session)
- main.py: app factory, lifespan (init_db + beets WAL enable), session
  middleware, minimal dashboard route

Verified boots end-to-end via TestClient: unauthenticated GET / redirects to
/auth/login, static files serve, and all 13 schema tables get created on
first run.

app/routers/dashboard.py

@@ -0,0 +1,16 @@
+from fastapi import APIRouter, Depends, Request
+from fastapi.templating import Jinja2Templates
+
+from app.security.deps import require_auth
+
+router = APIRouter(tags=["dashboard"])
+templates = Jinja2Templates(directory="app/templates")
+
+
+@router.get("/")
+async def dashboard(request: Request, user: dict = Depends(require_auth)):
+    return templates.TemplateResponse(
+        request,
+        "dashboard.html",
+        {"user": user},
+    )