Cluster B, milestone #99. TagService.merge_preview(source, target) computes the
same counts the apply produces (rule 93 parity) without mutating: images_moving
(source links the apply UPDATEs), images_already_on_target (links it drops),
source_total, series_pages, will_alias (_keep_as_alias), a kind/fandom
compatible flag (surfaced, not raised, so the UI can warn), and up to 6
thumbnails of the moving images. The admin /tags/<dest>/merge route gains a
dry_run flag returning the preview JSON.
Tests: preview moving-count == apply merged_count (parity), incompatible flagged
without raising, self/missing raise, admin dry_run returns preview + no mutation.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX
bvandeusen
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