feat: Add runtime DNS-rebinding protection for blocklist HTTP connections

## Problem
The blocklist URL validation at create/update time has a TOCTOU (time-of-check-to-time-of-use) window.
An attacker can perform a DNS-rebinding attack where:
1. User adds blocklist URL pointing to attacker.com
2. At create time, attacker.com resolves to a public IP → validation passes
3. Later, when fetching, attacker.com resolves to 192.168.1.1 (internal network)
4. HTTP client connects to the private IP, potentially accessing internal services

## Solution
Add runtime destination IP validation at connection time via a custom socket factory:

- Created 'dns_validated_connector.py' with create_dns_validated_socket_factory() that validates
  all resolved IPs before socket creation
- HTTP session now uses the validated socket factory, protecting all blocklist imports globally
- Rejects connections to RFC 1918 private ranges, loopback, link-local, ULA, multicast, and
  reserved addresses (IPv4 and IPv6)
- Added comprehensive test coverage with 13 test cases

## Changes
- backend/app/services/dns_validated_connector.py: Custom socket factory with IP validation
- backend/app/startup.py: Use DNS-validated socket factory in HTTP session creation
- backend/app/utils/ip_utils.py: Updated docstring explaining runtime validation
- backend/app/services/blocklist_downloader.py: Updated module docstring
- backend/app/services/blocklist_service.py: Updated docstrings explaining two-layer protection
- backend/tests/test_services/test_dns_validated_connector.py: Test suite for socket factory
- Docs/Architekture.md: Added detailed section on DNS-rebinding protection

## Testing
- All 13 DNS validation tests pass
- All blocklist downloader tests pass (unaffected by changes)
- Linting: ruff, mypy pass with --strict
- Test coverage: 90% line coverage on dns_validated_connector.py

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

backend/app/utils/ip_utils.py

@@ -141,10 +141,11 @@ async def validate_blocklist_url(url: str) -> None:
     - The hostname resolves to a public (non-private, non-reserved) IP address
     - IPv4-mapped IPv6 addresses are checked against IPv4 private ranges
 
-    Performs DNS resolution asynchronously to check the resolved IP.
-    This is a point-in-time check; DNS rebinding attacks may still be possible
-    at actual fetch time. Callers should re-validate the final connection
-    in the HTTP client layer.
+    Performs DNS resolution asynchronously to check the resolved IP. This is a
+    point-in-time check; the application uses a DNS-validated HTTP connector
+    that performs runtime re-validation at connection time to prevent DNS-rebinding
+    attacks where the same hostname resolves to a different (private) IP address
+    after this initial validation.
 
     Args:
         url: The blocklist URL to validate.