chore(db, model): simplify per hash DB lookup in copier (#10080)

This is a draft because I haven't adjusted all the tests yet, I'd like
to get feedback on the change overall first, before spending time on
that.

In my opinion the main win of this change is in it's lower complexity
resp. fewer moving parts. It should also be faster as it only does one
query instead of two, but I have no idea if that's practically
relevant.

This also mirrors the v1 DB, where a block map key had the name
appended. Not that this is an argument for the change, it was mostly
reassuring me that I might not be missing something key here
conceptually (I might still be of course, please tell me :) ).

And the change isn't mainly intrinsically motivated, instead it came
up while fixing a bug in the copier. And the nested nature of that code
makes the fix harder, and "un-nesting" it required me to understand
what's happening. This change fell out of that.

internal/db/sqlite/folderdb_local.go

@@ -99,7 +99,7 @@ func (s *folderDB) AllLocalBlocksWithHash(hash []byte) (iter.Seq[db.BlockMapEntr
 	// & blocklists is deferred (garbage collected) while the files list is
 	// not. This filters out blocks that are in fact deleted.
 	return iterStructs[db.BlockMapEntry](s.stmt(`
-		SELECT f.blocklist_hash as blocklisthash, b.idx as blockindex, b.offset, b.size FROM files f
+		SELECT f.blocklist_hash as blocklisthash, b.idx as blockindex, b.offset, b.size, f.name as filename FROM files f
 		LEFT JOIN blocks b ON f.blocklist_hash = b.blocklist_hash
 		WHERE f.device_idx = {{.LocalDeviceIdx}} AND b.hash = ?
 	`).Queryx(hash))