feat(memory): add working memory implementation (Issue #89)

Implements session-scoped ephemeral memory with:

Storage Backends:
- InMemoryStorage: Thread-safe fallback with TTL support and capacity limits
- RedisStorage: Primary storage with connection pooling and JSON serialization
- Auto-fallback from Redis to in-memory when unavailable

WorkingMemory Class:
- Key-value storage with TTL and reserved key protection
- Task state tracking with progress updates
- Scratchpad for reasoning steps with timestamps
- Checkpoint/snapshot support for recovery
- Factory methods for auto-configured storage

Tests:
- 55 unit tests covering all functionality
- Tests for basic ops, TTL, capacity, concurrency
- Tests for task state, scratchpad, checkpoints

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/memory/exceptions.py

@@ -94,6 +94,22 @@ class MemoryStorageError(MemoryError):
         self.backend = backend
 
 
+class MemoryConnectionError(MemoryError):
+    """Raised when memory storage connection fails."""
+
+    def __init__(
+        self,
+        message: str = "Memory connection failed",
+        *,
+        backend: str | None = None,
+        host: str | None = None,
+        **kwargs: Any,
+    ) -> None:
+        super().__init__(message, **kwargs)
+        self.backend = backend
+        self.host = host
+
+
 class MemorySerializationError(MemoryError):
     """Raised when memory serialization/deserialization fails."""