feat(cache): add cache store abstraction layer

Implement reusable cache store abstraction with in-memory and Redis
backends as foundation for prompt caching feature (PR1 of progressive delivery).

- Add CacheStore protocol defining cache interface
- Implement MemoryCacheStore with LRU, LFU, and TTL-only eviction policies
- Implement RedisCacheStore with connection pooling and retry logic
- Add CircuitBreaker for cache backend failure protection
- Include comprehensive unit tests (55 tests, >80% coverage)
- Add dependencies: cachetools>=5.5.0, redis>=5.2.0

This abstraction enables flexible caching implementations for the prompt
caching middleware without coupling to specific storage backends.

Signed-by: William Caban <willliam.caban@gmail.com>

uv.lock

@@ -1,5 +1,5 @@
 version = 1
-revision = 2
+revision = 3
 requires-python = ">=3.12"
 resolution-markers = [
     "(python_full_version >= '3.13' and platform_machine != 'aarch64' and sys_platform == 'linux') or (python_full_version >= '3.13' and sys_platform != 'darwin' and sys_platform != 'linux')",
@@ -1996,6 +1996,7 @@ dependencies = [
     { name = "aiohttp" },
     { name = "aiosqlite" },
     { name = "asyncpg" },
+    { name = "cachetools" },
     { name = "fastapi" },
     { name = "fire" },
     { name = "h11" },
@@ -2013,6 +2014,7 @@ dependencies = [
     { name = "python-dotenv" },
     { name = "python-multipart" },
     { name = "pyyaml" },
+    { name = "redis" },
     { name = "rich" },
     { name = "sqlalchemy", extra = ["asyncio"] },
     { name = "starlette" },
@@ -2147,6 +2149,7 @@ requires-dist = [
     { name = "aiohttp" },
     { name = "aiosqlite", specifier = ">=0.21.0" },
     { name = "asyncpg" },
+    { name = "cachetools", specifier = ">=5.5.0" },
     { name = "fastapi", specifier = ">=0.115.0,<1.0" },
     { name = "fire" },
     { name = "h11", specifier = ">=0.16.0" },
@@ -2166,6 +2169,7 @@ requires-dist = [
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     { name = "pyyaml", specifier = ">=6.0" },
     { name = "pyyaml", specifier = ">=6.0.2" },
+    { name = "redis", specifier = ">=5.2.0" },
     { name = "rich" },
     { name = "sqlalchemy", extras = ["asyncio"], specifier = ">=2.0.41" },
     { name = "starlette" },
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