scanAndSyncStorage builds sourcesInQdrant and embeddableFiles in the
same pass but only ever checks one direction - is this on-disk file
already embedded? It never checks the reverse, so points left behind
by a deleted or replaced ZIM just sit in Qdrant forever and keep
showing up as search hits for content that doesn't exist anymore.
Added a reverse sweep that diffs the two sets and purges anything left
in Qdrant with no file backing it. Guarded so it's a no-op when the
disk scan comes back empty, so a filesystem hiccup can't get misread
as "delete everything."
Also has to exclude README.md/docs, since those get embedded by
discoverNomadDocs and live outside the kb_uploads/zim paths the
scanner actually walks. First pass without that exclusion would have
wiped the docs KB on every single sync - caught it by testing against
a live install where those sources were already indexed.
Verified live: found exactly one real orphan out of 29 indexed
sources (an old devdocs_en_bash zim superseded by a newer download),
purged it, and left the other 28 - including all the docs - alone.
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