HTTP, S3, and WebDAV sources all check `revision is not None and
snapshot.get(uri) == revision` to decide UPSERT vs UNCHANGED. When
a server returns no ETag or Last-Modified, revision is None and the
condition always fails — every sweep emits UPSERT even though the
content hasn't changed.
Now emit UNCHANGED when revision is None and the URI is already
known (has been ingested before). A first-time discovery with no
revision still correctly emits UPSERT.
