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_authn/test_webauthn.py

@@ -1,5 +1,6 @@
 import os
 
+import pytest
 from cryptography.hazmat.primitives.asymmetric import ec
 from cryptography.hazmat.primitives.hashes import SHA256
 from fido2.cose import ES256
@@ -233,8 +234,6 @@ def test_complete_authentication_verifies_signature() -> None:
 
 
 def test_complete_authentication_wrong_signature_raises() -> None:
-    import pytest
-
     service = _make_service()
     _private_key, credential_id, attested = _generate_credential()