Extract cache/backend.py to mizan-core (Tier A)

cache/backend.py is pure framework-agnostic key-value abstraction —
CacheBackend Protocol, MemoryCache, RedisCache. No Django imports.
Moves to cores/mizan-python/src/mizan_core/cache/backend.py with no
content changes; mizan-django re-imports.

Verified: mizan-core 15/15, mizan-django 348 pass / 21 skip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backends/mizan-django/src/mizan/cache/__init__.py

@@ -15,7 +15,7 @@ import logging
 import threading
 from typing import Any
 
-from .backend import CacheBackend, MemoryCache, RedisCache
+from mizan_core.cache.backend import CacheBackend, MemoryCache, RedisCache
 from mizan_core.cache.keys import derive_cache_key, CONTEXT_KEY_PREFIX
 
 logger = logging.getLogger("mizan.cache")

backends/mizan-django/src/mizan/tests/test_core.py

@@ -2782,7 +2782,7 @@ class CacheBackendTests(TestCase):
     """Tests for MemoryCache backend operations."""
 
     def setUp(self):
-        from mizan.cache.backend import MemoryCache
+        from mizan_core.cache.backend import MemoryCache
         self.cache = MemoryCache()
 
     def test_get_miss(self):
@@ -2825,7 +2825,7 @@ class CachePurgeTests(TestCase):
 
     def setUp(self):
         from mizan.cache import cache_put, set_cache
-        from mizan.cache.backend import MemoryCache
+        from mizan_core.cache.backend import MemoryCache
 
         self.cache = MemoryCache()
         set_cache(self.cache)
@@ -2869,7 +2869,7 @@ class CacheIntegrationTests(TestCase):
         self.factory = RequestFactory()
 
         from mizan.cache import set_cache
-        from mizan.cache.backend import MemoryCache
+        from mizan_core.cache.backend import MemoryCache
         from mizan.setup.settings import clear_settings_cache
 
         self.cache = MemoryCache()
@@ -3139,7 +3139,7 @@ class CachePolicyIntegrationTests(TestCase):
         """Context with mixed revs uses the maximum for cache key."""
         from mizan_core.cache.keys import derive_cache_key
         from mizan.cache import set_cache, reset_cache
-        from mizan.cache.backend import MemoryCache
+        from mizan_core.cache.backend import MemoryCache
         from mizan.setup.settings import clear_settings_cache
         from django.test import override_settings
 
@@ -3292,7 +3292,7 @@ class RedisCacheBackendTests(TestCase):
     def setUp(self):
         if _SKIP_REDIS:
             self.skipTest(_SKIP_MSG)
-        from mizan.cache.backend import RedisCache
+        from mizan_core.cache.backend import RedisCache
         self.cache = RedisCache(REDIS_URL, prefix="mizan:test:")
         self.cache.clear()
 
@@ -3370,7 +3370,7 @@ class RedisCachePurgeTests(TestCase):
     def setUp(self):
         if _SKIP_REDIS:
             self.skipTest(_SKIP_MSG)
-        from mizan.cache.backend import RedisCache
+        from mizan_core.cache.backend import RedisCache
         from mizan.cache import cache_put
         self.cache = RedisCache(REDIS_URL, prefix="mizan:test:")
         self.cache.clear()