feat(backend): record tag usage in filters to activity.tag_uses

Listing files with a tag filter now logs each referenced tag to
activity.tag_uses, flagging it included (positive) or excluded (negated
under an odd number of NOTs); the untagged pseudo-token is skipped. The
filter AST is reused to determine polarity, so grouped negations like
!(A|B) mark both tags excluded.

Recording happens only when a filter is first applied — not on cursor
pagination or an anchored return — so one browse counts once. The write
is best-effort and never fails the listing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/port/repository.go

@@ -62,6 +62,10 @@ type FileRepo interface {
 
 	// RecordView appends a view-history row (activity.file_views) for the user.
 	RecordView(ctx context.Context, fileID uuid.UUID, userID int16) error
+	// RecordTagUses logs the tags referenced in a filter DSL to
+	// activity.tag_uses, flagging each included or excluded. Best-effort
+	// analytics — callers may ignore the error.
+	RecordTagUses(ctx context.Context, userID int16, filterDSL string) error
 }
 
 // TagRepo is the persistence interface for tags.