feat(psa): add in-memory TTL cache for board statuses

Add PSACache class with get/set/invalidate/clear operations and TTL expiry.
Board statuses are cached for 1 hour in the provider.
Cache is cleared when a PSA connection is re-tested.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/psa/cache.py

@@ -0,0 +1,38 @@
+"""Simple in-memory TTL cache for PSA API responses."""
+from __future__ import annotations
+
+import time
+from typing import Any
+
+
+class PSACache:
+    """Account-scoped in-memory cache with TTL expiry."""
+
+    def __init__(self) -> None:
+        self._store: dict[str, tuple[Any, float]] = {}
+
+    def get(self, key: str) -> Any | None:
+        entry = self._store.get(key)
+        if entry is None:
+            return None
+        value, expires_at = entry
+        if time.time() > expires_at:
+            del self._store[key]
+            return None
+        return value
+
+    def set(self, key: str, value: Any, ttl_seconds: int) -> None:
+        self._store[key] = (value, time.time() + ttl_seconds)
+
+    def invalidate(self, prefix: str) -> None:
+        """Remove all entries matching a key prefix."""
+        keys_to_remove = [k for k in self._store if k.startswith(prefix)]
+        for k in keys_to_remove:
+            del self._store[k]
+
+    def clear(self) -> None:
+        self._store.clear()
+
+
+# Global singleton — acceptable at current scale (see design doc section 6)
+psa_cache = PSACache()