Implement global rate limiter and refactor auth middleware

- Add global rate limiter utility with configurable limits and cleanup
- Move rate limiting logic to middleware for consistent application
- Update auth routes to use new rate limiter
- Add comprehensive tests for rate limiter functionality
- Update documentation with backend development guidelines and tasks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/exceptions.py

@@ -39,7 +39,6 @@ See Backend-Development.md for the complete exception contract.
 
 from __future__ import annotations
 
-
 # ---------------------------------------------------------------------------
 # Exception Base Classes (Categories)
 # ---------------------------------------------------------------------------
@@ -107,6 +106,19 @@ class RateLimitError(DomainError):
 
     error_code: str = "rate_limit_exceeded"
 
+    def __init__(self, message: str, retry_after_seconds: float = 60.0) -> None:
+        """Initialize with a message and optional retry-after time.
+
+        Args:
+            message: Description of the rate limit violation.
+            retry_after_seconds: Estimated seconds to wait before retrying (default 60).
+        """
+        self.retry_after_seconds: float = retry_after_seconds
+        super().__init__(message)
+
+    def get_error_metadata(self) -> dict[str, str | int | float | bool | None]:
+        return {"retry_after_seconds": self.retry_after_seconds}
+
 
 # ---------------------------------------------------------------------------
 # Jail-Specific Exceptions