refactor(admin): consolidate Tier-A dry-run/apply handlers onto one helper (#753)

DRY pass on the cleanup/admin destructive-ops surface (task #753, hardened
process #594). Five Tier-A endpoints repeated the same get_json -> dry_run ->
run_sync(service_fn) -> jsonify block verbatim. Extract _run_dry_run_op(service_fn,
**kwargs); the five route handlers now delegate. reconcile keeps its source_id
validation and passes it through **kwargs.

The cleanup_service predicates were already shared between preview and apply
(find_*_conditions / find_duplicate_post_groups) — the post-data-loss fix — so no
backend-logic change; this is purely the HTTP-handler boilerplate.

Consumers (all routed through the helper, verified no copy left behind):
  prune_unused_tags, prune_bare_posts, reconcile_duplicate_posts (+source_id),
  purge_legacy_tags, reset_content_tagging.

Added route-level tests for prune-bare (apply) and reconcile (apply + source_id
passthrough + invalid-source_id 400) — the two helper consumers that previously
had only service-level coverage, so every consumer is exercised at the route.

Findings B (queued-response helper) and C (store dry-run POST helper) identified
but not applied this pass (operator scoped to A). The card preview->commit state
machine is deferred to a frontend pattern-consistency sweep.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/admin.py

@@ -39,6 +39,23 @@ def _bulk_image_confirm_token(image_ids: list[int]) -> str:
     return digest[:8]
 
 
+async def _run_dry_run_op(service_fn, **service_kwargs):
+    """Shared body for the Tier-A dry-run/apply endpoints: read the `dry_run`
+    flag, run the cleanup_service predicate under `run_sync`, and return its
+    result dict. The SAME `service_fn` drives both preview and apply (the flag
+    just toggles), so a handler physically can't let its preview diverge from
+    its delete (rule 93). Default False preserves the existing contract — the UI
+    always passes `dry_run` explicitly (true to preview, false to apply). Extra
+    service kwargs (e.g. `source_id`) pass straight through."""
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", False))
+    async with get_session() as session:
+        result = await session.run_sync(
+            lambda sync_sess: service_fn(sync_sess, dry_run=dry_run, **service_kwargs)
+        )
+    return jsonify(result)
+
+
 @admin_bp.route("/artists/<slug>/cascade-delete", methods=["POST"])
 async def artist_cascade_delete(slug: str):
     body = await request.get_json(silent=True) or {}
@@ -193,16 +210,7 @@ async def tags_prune_unused():
     re-call with dry_run=false."""
     from ..services.cleanup_service import prune_unused_tags
 
-    body = await request.get_json(silent=True) or {}
-    dry_run = bool(body.get("dry_run", False))
-
-    async with get_session() as session:
-        result = await session.run_sync(
-            lambda sync_sess: prune_unused_tags(
-                sync_sess, dry_run=dry_run,
-            )
-        )
-    return jsonify(result)
+    return await _run_dry_run_op(prune_unused_tags)
 
 
 @admin_bp.route("/posts/prune-bare", methods=["POST"])
@@ -214,16 +222,7 @@ async def posts_prune_bare():
     prune itself, so the preview can't diverge from the delete."""
     from ..services.cleanup_service import prune_bare_posts
 
-    body = await request.get_json(silent=True) or {}
-    dry_run = bool(body.get("dry_run", False))
-
-    async with get_session() as session:
-        result = await session.run_sync(
-            lambda sync_sess: prune_bare_posts(
-                sync_sess, dry_run=dry_run,
-            )
-        )
-    return jsonify(result)
+    return await _run_dry_run_op(prune_bare_posts)
 
 
 @admin_bp.route("/posts/reconcile-duplicates", methods=["POST"])
@@ -237,20 +236,13 @@ async def posts_reconcile_duplicates():
     from ..services.cleanup_service import reconcile_duplicate_posts
 
     body = await request.get_json(silent=True) or {}
-    dry_run = bool(body.get("dry_run", False))
     raw_source = body.get("source_id")
     try:
         source_id = int(raw_source) if raw_source is not None else None
     except (TypeError, ValueError):
         return _bad("invalid_source_id", detail="source_id must be an integer")
 
-    async with get_session() as session:
-        result = await session.run_sync(
-            lambda sync_sess: reconcile_duplicate_posts(
-                sync_sess, source_id=source_id, dry_run=dry_run,
-            )
-        )
-    return jsonify(result)
+    return await _run_dry_run_op(reconcile_duplicate_posts, source_id=source_id)
 
 
 @admin_bp.route("/tags/purge-legacy", methods=["POST"])
@@ -263,14 +255,7 @@ async def tags_purge_legacy():
     operator confirms with dry_run=false."""
     from ..services.cleanup_service import purge_legacy_tags
 
-    body = await request.get_json(silent=True) or {}
-    dry_run = bool(body.get("dry_run", False))
-
-    async with get_session() as session:
-        result = await session.run_sync(
-            lambda sync_sess: purge_legacy_tags(sync_sess, dry_run=dry_run)
-        )
-    return jsonify(result)
+    return await _run_dry_run_op(purge_legacy_tags)
 
 
 @admin_bp.route("/tags/reset-content", methods=["POST"])
@@ -284,14 +269,7 @@ async def tags_reset_content():
     Irreversible except via DB backup restore."""
     from ..services.cleanup_service import reset_content_tagging
 
-    body = await request.get_json(silent=True) or {}
-    dry_run = bool(body.get("dry_run", False))
-
-    async with get_session() as session:
-        result = await session.run_sync(
-            lambda sync_sess: reset_content_tagging(sync_sess, dry_run=dry_run)
-        )
-    return jsonify(result)
+    return await _run_dry_run_op(reset_content_tagging)
 
 
 @admin_bp.route("/tags/normalize", methods=["POST"])