fix(slice5): validate raw metadata input + prune stale bulk selections

Code-review follow-ups:
- Validate the raw submitted value before apply_values coerces it, in both
  the single-edit and bulk endpoints — a scalar set onto a tags field now
  rejects (422 / rejected) instead of silently char-splitting into a list.
- Catalog prunes bulk selections to the currently-visible (filtered) entries
  after each list fetch, so the bulk bar can't act on rows the user has
  filtered out of view.

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

backend/app/api_metadata.py

@@ -89,11 +89,13 @@ def make_router(config: Config, gitea: Gitea, bot: Bot) -> APIRouter:
         if st is None:
             raise HTTPException(404, f"{md_path} not found")
 
-        new_entry = metadata_mod.apply_values(st.entry, body.values)
-        problems = metadata_schema.validate(
-            metadata_mod.metadata_dict(new_entry), fields)
+        # Validate the *raw* submitted values (INV-4): catch a type mismatch
+        # before `apply_values` coerces it — e.g. a scalar handed to a `tags`
+        # field would otherwise char-split into a valid-looking list.
+        problems = metadata_schema.validate(body.values, fields)
         if problems:
             raise HTTPException(422, {"problems": [p.as_dict() for p in problems]})
+        new_entry = metadata_mod.apply_values(st.entry, body.values)
 
         files = metadata_mod.write_entry_files(md_path, new_entry, st)
         try:
@@ -153,14 +155,15 @@ def make_router(config: Config, gitea: Gitea, bot: Bot) -> APIRouter:
                 rejected.append({"slug": slug, "reason": "not found"})
                 continue
             new_value = _apply_op(st.entry, body.op, body.field, body.value)
-            new_entry = metadata_mod.apply_values(st.entry, {body.field: new_value})
-            problems = metadata_schema.validate(
-                metadata_mod.metadata_dict(new_entry), fields)
+            # Validate the *raw* new value before coercion (see edit_meta) so a
+            # scalar `set` onto a tags field is rejected, not char-split.
+            problems = metadata_schema.validate({body.field: new_value}, fields)
             if problems:
                 rejected.append({"slug": slug,
                                  "reason": "; ".join(p.message for p in problems)})
                 continue
             applied.append(slug)
+            new_entry = metadata_mod.apply_values(st.entry, {body.field: new_value})
             if metadata_mod.metadata_dict(new_entry) != metadata_mod.metadata_dict(st.entry):
                 all_ops.extend(metadata_mod.write_entry_files(md_path, new_entry, st))