feat: add snapshot cache and state file I/O methods

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/snapshot_downloader.py

@@ -539,6 +539,39 @@ class SnapshotDownloader:
             return None
 
 
+    def load_snapshot_cache(self) -> List[Dict[str, Any]]:
+        cache_file = self.output_dir / "snapshots_cache.json"
+        if not cache_file.exists():
+            return []
+        try:
+            with open(cache_file, "r", encoding="utf-8") as f:
+                data = json.load(f)
+            return data if isinstance(data, list) else []
+        except (json.JSONDecodeError, OSError):
+            self.logger.warning("Could not read snapshot cache; starting fresh")
+            return []
+
+    def save_snapshot_cache(self, snapshots: List[Dict[str, Any]]) -> None:
+        cache_file = self.output_dir / "snapshots_cache.json"
+        with open(cache_file, "w", encoding="utf-8") as f:
+            json.dump(snapshots, f, indent=2, default=str)
+
+    def load_last_run_date(self) -> Optional[str]:
+        state_file = self.output_dir / "last_run.json"
+        if not state_file.exists():
+            return None
+        try:
+            with open(state_file, "r", encoding="utf-8") as f:
+                data = json.load(f)
+            return data.get("last_date_to")
+        except (json.JSONDecodeError, OSError):
+            return None
+
+    def save_last_run_date(self, date: str) -> None:
+        state_file = self.output_dir / "last_run.json"
+        with open(state_file, "w", encoding="utf-8") as f:
+            json.dump({"last_date_to": date}, f)
+
     async def generate_html_file(
         self, snapshots: List[Dict[str, Any]], date_from: str, date_to: str
     ) -> Path: