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>

cores/mizan-python/src/mizan_core/cache/backend.py

@@ -0,0 +1,115 @@
+"""
+Cache backends — MemoryCache (testing) and RedisCache (production).
+
+Simple key-value stores. No reverse indexes. Cache keys are derived
+from HMAC, so scoped purge just recomputes the key and deletes it.
+Broad purge uses key-prefix scan (rare operation).
+"""
+
+from __future__ import annotations
+
+from typing import Protocol
+
+
+class CacheBackend(Protocol):
+    """Interface that all Mizan cache backends implement."""
+
+    def get(self, key: str) -> bytes | None: ...
+    def set(self, key: str, value: bytes) -> None: ...
+    def delete(self, key: str) -> bool: ...
+    def delete_by_prefix(self, prefix: str) -> int: ...
+    def clear(self) -> None: ...
+
+
+class MemoryCache:
+    """
+    In-memory cache backend for testing.
+
+    Uses a Python dict. No persistence, no cross-process sharing.
+    """
+
+    def __init__(self) -> None:
+        self._store: dict[str, bytes] = {}
+
+    def get(self, key: str) -> bytes | None:
+        return self._store.get(key)
+
+    def set(self, key: str, value: bytes) -> None:
+        self._store[key] = value
+
+    def delete(self, key: str) -> bool:
+        if key in self._store:
+            del self._store[key]
+            return True
+        return False
+
+    def delete_by_prefix(self, prefix: str) -> int:
+        to_delete = [k for k in self._store if k.startswith(prefix)]
+        for k in to_delete:
+            del self._store[k]
+        return len(to_delete)
+
+    def clear(self) -> None:
+        self._store.clear()
+
+
+class RedisCache:
+    """
+    Redis-backed cache backend for production.
+
+    Simple GET/SET/DEL. No reverse indexes. Scoped purge recomputes
+    the HMAC key and deletes directly. Broad purge uses SCAN.
+    """
+
+    DEFAULT_TTL = 86400  # 24h safety-net
+
+    def __init__(
+        self,
+        redis_url: str,
+        prefix: str = "mizan:",
+        ttl: int | None = None,
+    ) -> None:
+        try:
+            import redis as redis_lib
+        except ImportError:
+            raise ImportError(
+                "Redis is required for Mizan's cache backend. "
+                "Install it with: pip install mizan[cache]"
+            )
+        self._client = redis_lib.from_url(
+            redis_url,
+            socket_connect_timeout=5,
+            socket_timeout=5,
+            health_check_interval=30,
+            retry_on_timeout=True,
+            max_connections=50,
+        )
+        self._prefix = prefix
+        self._ttl = ttl if ttl is not None else self.DEFAULT_TTL
+
+    def _key(self, key: str) -> str:
+        return f"{self._prefix}{key}"
+
+    def get(self, key: str) -> bytes | None:
+        return self._client.get(self._key(key))
+
+    def set(self, key: str, value: bytes) -> None:
+        self._client.set(self._key(key), value, ex=self._ttl)
+
+    def delete(self, key: str) -> bool:
+        return self._client.unlink(self._key(key)) > 0
+
+    def delete_by_prefix(self, prefix: str) -> int:
+        pattern = f"{self._prefix}{prefix}*"
+        count = 0
+        cursor = 0
+        while True:
+            cursor, keys = self._client.scan(cursor, match=pattern, count=1000)
+            if keys:
+                count += self._client.unlink(*keys)
+            if cursor == 0:
+                break
+        return count
+
+    def clear(self) -> None:
+        self.delete_by_prefix("")