feat(tags): non-mutating merge preview + admin dry_run (#8a)

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

backend/app/api/admin.py

@@ -171,6 +171,30 @@ async def tag_merge(dest_id: int):
     if not isinstance(source_id, int) or source_id == dest_id:
         return _bad("invalid_source_id", detail="source_id must be int and differ from dest")
 
+    # dry_run: non-mutating preview (counts + sample) so the operator can
+    # confirm the target before the irreversible merge (#8, rule 93 parity).
+    if body.get("dry_run"):
+        async with get_session() as session:
+            try:
+                p = await TagService(session).merge_preview(
+                    source_id=source_id, target_id=dest_id,
+                )
+            except TagValidationError as exc:
+                return _bad("tag_not_found", status=404, detail=str(exc))
+        return jsonify({
+            "preview": {
+                "source_id": p.source_id, "source_name": p.source_name,
+                "target_id": p.target_id, "target_name": p.target_name,
+                "compatible": p.compatible,
+                "images_moving": p.images_moving,
+                "images_already_on_target": p.images_already_on_target,
+                "source_total": p.source_total,
+                "series_pages": p.series_pages,
+                "will_alias": p.will_alias,
+                "sample_thumbnails": p.sample_thumbnails,
+            },
+        })
+
     async with get_session() as session:
         try:
             result = await TagService(session).merge(