refactor(backend): migrate type checking from mypy to pyright

Replace mypy>=1.8.0 with pyright>=1.1.390. Remove all [tool.mypy] and
[tool.pydantic-mypy] sections from pyproject.toml and add
pyrightconfig.json (standard mode, SQLAlchemy false-positive rules
suppressed globally).

Fixes surfaced by pyright:
- Remove unreachable except AuthError clauses in login/login_oauth (same class as AuthenticationError)
- Fix Pydantic v2 list Field: min_items/max_items → min_length/max_length
- Split OAuthProviderConfig TypedDict into required + optional(email_url) inheritance
- Move JWTError/ExpiredSignatureError from lazy try-block imports to module level
- Add timezone-aware guard to UserSession.is_expired to match sibling models
- Fix is_active: bool → bool | None in three organization repo signatures
- Initialize search_filter = None before conditional block (possibly unbound fix)
- Add bool() casts to model is_expired and repo is_active/is_superuser returns
- Restructure except (JWTError, Exception) into separate except clauses

backend/app/core/exceptions.py

@@ -222,7 +222,7 @@ async def http_exception_handler(request: Request, exc: HTTPException) -> JSONRe
     )
 
     error_response = ErrorResponse(
-        errors=[ErrorDetail(code=error_code, message=str(exc.detail))]
+        errors=[ErrorDetail(code=error_code, message=str(exc.detail), field=None)]
     )
 
     return JSONResponse(
@@ -254,7 +254,7 @@ async def unhandled_exception_handler(request: Request, exc: Exception) -> JSONR
         message = f"{type(exc).__name__}: {exc!s}"
 
     error_response = ErrorResponse(
-        errors=[ErrorDetail(code=ErrorCode.INTERNAL_ERROR, message=message)]
+        errors=[ErrorDetail(code=ErrorCode.INTERNAL_ERROR, message=message, field=None)]
     )
 
     return JSONResponse(