fix: resolve all ruff lint errors and type checker warnings

- Use Annotated[str, Form()] for FastAPI dependencies (FAST002)
- Add missing type annotations across src/ and tests/ (ANN001/003/201/202)
- Reduce function arguments via request.form() reads (PLR0913)
- Combine return paths to reduce return statements (PLR0911)
- Use anyio.Path for async-safe filesystem operations (ASYNC240)
- Extract constants, helpers, and dict comprehensions for clarity
- Move inline imports to top-level (PLC0415)
- Use raw strings for regex match patterns (RUF043)
- Fix redundant get_session_user call in delete_user (not-iterable)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_admin/test_admin_routes.py

@@ -1,3 +1,4 @@
+from base64 import urlsafe_b64encode
 from datetime import UTC, datetime
 
 import pytest
@@ -46,7 +47,7 @@ async def _login(
     )
 
 
-async def _create_target_user(
+async def _create_target_user(  # noqa: PLR0913
     client: AsyncClient,
     *,
     userid: str = "target-user-01",
@@ -365,8 +366,6 @@ async def test_delete_webauthn_credential(client: AsyncClient) -> None:
     )
 
     # URL uses base64url without padding
-    from base64 import urlsafe_b64encode
-
     credential_id_b64 = urlsafe_b64encode(credential_id).decode().rstrip("=")
 
     token = await get_csrf_token(client)