fix(auth): Casdoor JWT verification and 401 semantics; add /auth/login password-grant endpoint

- verify kid from the JWT header (pyjwt decode() returns claims, not the
  header — the old lookup was always None and rejected every token)
- convert JWK entries to crypto keys explicitly (pyjwt 2.13 dropped dict
  keys) and fetch the JWKS from /.well-known/jwks
- 401 for missing/invalid/expired/wrong-issuer/wrong-audience tokens
  (was 403/400); 403 stays for valid-but-roleless users; 503 when Casdoor
  is unreachable; new CASDOOR_ISSUER setting separates the in-network
  endpoint from the browser-facing issuer
- dev-header fallback in inventory/pm scopes now requires an explicit
  X-User-Role header (no auth info at all -> 401)
- new POST /api/v1/auth/login: password grant against Casdoor, local JWT
  verification, auto-provisioning, returns token + profile with the role
  resolved from the app users table

backend/app/routers/__init__.py

@@ -1,6 +1,7 @@
 """All API routers, re-exported for main.py assembly."""
 from app.routers import (
     attribute_definitions,
+    auth,
     brand_materials,
     brands,
     category_attributes,
@@ -29,6 +30,7 @@ from app.routers import (
 )
 
 ROUTERS = [
+    auth.router,
     units.router,
     brands.router,
     material_categories.router,