feat(rss): support deleted-file sync (#14493)

### What problem does this PR solve?

Partially addresses #14362.

This PR enables syncing deleted files for RSS data sources.

Previously, RSS incremental sync only returned feed entries whose
timestamps were inside the poll window. If an entry was removed from the
RSS feed, RAGFlow had no full current RSS snapshot to pass into the
shared stale-document cleanup path, so the deleted remote entry could
remain in the knowledge base.

This PR:
- adds `retrieve_all_slim_docs_perm_sync()` to `RSSConnector`
- reuses the same `rss:<md5(stable_key)>` document ID derivation used by
normal RSS ingest
- returns `(document_generator, file_list)` for incremental RSS sync
when `sync_deleted_files` is enabled
- captures the poll end timestamp before snapshot/poll so cleanup does
not race against the same sync window
- adds start/end logs around RSS slim snapshot collection
- exposes the deleted-file sync toggle for RSS in the data source UI

Per maintainer request on related datasource PRs, this PR contains no
test-case changes. Local verification was run with an external script.

Validation:
- `uv run ruff check common/data_source/rss_connector.py
rag/svr/sync_data_source.py`
- `uv run pytest test/unit_test/rag/test_sync_data_source.py -q`
- `./node_modules/.bin/eslint
src/pages/user-setting/data-source/constant/index.tsx`
- `git diff --check`
- `uv run python /tmp/verify_rss_deleted_sync.py --repo
/root/74/ragflow`

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

common/data_source/rss_connector.py

@@ -10,14 +10,20 @@ import feedparser
 import requests
 
 from common.data_source.config import INDEX_BATCH_SIZE, REQUEST_TIMEOUT_SECONDS, DocumentSource
-from common.data_source.interfaces import LoadConnector, PollConnector
-from common.data_source.models import Document, GenerateDocumentsOutput, SecondsSinceUnixEpoch
+from common.data_source.interfaces import LoadConnector, PollConnector, SlimConnectorWithPermSync
+from common.data_source.models import (
+    Document,
+    GenerateDocumentsOutput,
+    GenerateSlimDocumentOutput,
+    SecondsSinceUnixEpoch,
+    SlimDocument,
+)
 from common.ssrf_guard import assert_url_is_safe, pin_dns as _pin_dns
 
 _MAX_REDIRECTS = 10
 
 
-class RSSConnector(LoadConnector, PollConnector):
+class RSSConnector(LoadConnector, PollConnector, SlimConnectorWithPermSync):
     def __init__(self, feed_url: str, batch_size: int = INDEX_BATCH_SIZE) -> None:
         self.feed_url = feed_url.strip()
         self.batch_size = batch_size
@@ -40,6 +46,25 @@ class RSSConnector(LoadConnector, PollConnector):
     def poll_source(self, start: SecondsSinceUnixEpoch, end: SecondsSinceUnixEpoch) -> GenerateDocumentsOutput:
         yield from self._load_entries(start=start, end=end)
 
+    def retrieve_all_slim_docs_perm_sync(
+        self,
+        callback: Any = None,
+    ) -> GenerateSlimDocumentOutput:
+        del callback
+
+        feed = self._read_feed(require_entries=False)
+        batch: list[SlimDocument] = []
+
+        for entry in feed.entries:
+            batch.append(SlimDocument(id=self._build_document_id(entry)))
+
+            if len(batch) >= self.batch_size:
+                yield batch
+                batch = []
+
+        if batch:
+            yield batch
+
     def _load_entries(
         self,
         start: SecondsSinceUnixEpoch | None = None,
@@ -130,7 +155,7 @@ class RSSConnector(LoadConnector, PollConnector):
     def _build_document(self, entry: Any, updated_at: datetime) -> Document:
         link = (entry.get("link") or "").strip()
         title = (entry.get("title") or "").strip()
-        stable_key = (entry.get("id") or link or title or self.feed_url).strip()
+        stable_key = self._resolve_stable_key(entry)
         semantic_identifier = title or link or stable_key
         content = self._build_content(entry, semantic_identifier)
         blob = content.encode("utf-8")
@@ -152,7 +177,7 @@ class RSSConnector(LoadConnector, PollConnector):
             metadata["categories"] = categories
 
         return Document(
-            id=f"rss:{hashlib.md5(stable_key.encode('utf-8')).hexdigest()}",
+            id=self._build_document_id(entry),
             source=DocumentSource.RSS,
             semantic_identifier=semantic_identifier,
             extension=".txt",
@@ -180,6 +205,15 @@ class RSSConnector(LoadConnector, PollConnector):
 
         return "\n\n".join(part for part in parts if part).strip()
 
+    def _build_document_id(self, entry: Any) -> str:
+        stable_key = self._resolve_stable_key(entry)
+        return f"rss:{hashlib.md5(stable_key.encode('utf-8')).hexdigest()}"
+
+    def _resolve_stable_key(self, entry: Any) -> str:
+        link = (entry.get("link") or "").strip()
+        title = (entry.get("title") or "").strip()
+        return (entry.get("id") or link or title or self.feed_url).strip()
+
     def _resolve_entry_time(self, entry: Any) -> datetime:
         for field in ("updated_parsed", "published_parsed"):
             value = entry.get(field)