Add subscription mode, global cross-run URL store and --assume-done import

- dlphelper/globalstore.py: ~/.local/share/dlphelper/urls.json records every
  successfully downloaded URL once, regardless of OUT_DIR - reposts in other
  channels and re-runs of moved channels no longer re-download
- --subs subscriptions.json: batch mode over a configurable channel list;
  re-exports each channel by default (new links get picked up), --no-fresh
  reuses cached exports
- --assume-done <file>: manual import step that marks old URL lists as done
  in the global store (deliberate, unverified, never automatic)
- lazy downloader startup: no gofile guest account / no curl runs when all
  URLs are already done; token prompt skipped in --urls-file mode
- write_failed_urls creates OUT_DIR if needed

dlphelper/globalstore.py

@@ -0,0 +1,70 @@
+"""Cross-run, cross-channel record of completed URLs.
+
+This is what prevents re-downloading your collection: state.json only knows
+about one OUT_DIR, but the same gofile link posted in a second channel would
+otherwise be downloaded again into that channel's OUT_DIR. The global store
+(default: ~/.local/share/dlphelper/urls.json) records every successfully
+downloaded URL once, forever, regardless of OUT_DIR.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import os
+import time
+from pathlib import Path
+
+log = logging.getLogger(__name__)
+
+
+def default_global_path() -> Path:
+    data_home = os.environ.get("XDG_DATA_HOME") or str(
+        Path.home() / ".local" / "share"
+    )
+    return Path(data_home) / "dlphelper" / "urls.json"
+
+
+class GlobalStore:
+    """JSON file mapping url -> {status, channel, host, files, updated}."""
+
+    def __init__(self, path: Path | None = None):
+        self.path = path
+        self.records: dict[str, dict] = {}
+        if self.path and self.path.exists():
+            try:
+                self.records = json.loads(self.path.read_text(encoding="utf-8"))
+                if not isinstance(self.records, dict):
+                    raise ValueError("global store root must be an object")
+            except (ValueError, OSError) as exc:
+                backup = self.path.with_suffix(".json.bak")
+                log.warning("Corrupt global store %s (%s); backed up to %s",
+                            self.path, exc, backup)
+                self.path.replace(backup)
+                self.records = {}
+
+    def is_done(self, url: str) -> bool:
+        if not self.records:
+            return False
+        rec = self.records.get(url)
+        return bool(rec) and rec.get("status") == "ok"
+
+    def mark(self, url: str, **fields) -> None:
+        if self.path is None:
+            return
+        rec = self.records.setdefault(url, {"status": "ok"})
+        for key, value in fields.items():
+            if value is not None:
+                rec[key] = value
+        rec["status"] = "ok"
+        rec["updated"] = time.strftime("%Y-%m-%dT%H:%M:%S")
+        self.save()
+
+    def save(self) -> None:
+        if self.path is None:
+            return
+        self.path.parent.mkdir(parents=True, exist_ok=True)
+        tmp = self.path.with_suffix(".json.tmp")
+        tmp.write_text(json.dumps(self.records, indent=2, sort_keys=True),
+                       encoding="utf-8")
+        tmp.replace(self.path)