feat(tags): system-tag UI markers + full protection sweep (step 4 of #128)

UI: shield marker + tooltip on TagChip and TagCard; system tags hide
rename/merge/delete affordances (chip kebab entirely — set-fandom never
applies to their general kind; remove stays, un-tagging is normal use).
Aliases stay available: mapping model outputs ONTO a system tag is
useful. Directory cards carry is_system.

Every destructive path that could take out a system row is now guarded,
found by sweeping run 1891s off-by-three failures — each one was a
surface that would have eaten the seeded tags:
- prune-unused: predicate exempts is_system (they ship with zero
  applications and matched every unused condition)
- reset-content: predicate exempts is_system AND keeps their
  applications — hygiene flags describe the file, not content tagging
- admin tag DELETE: refused with system_tag error
- normalize_existing_tags: scan excludes is_system — canonicalization
  would recase wip -> Wip behind TagService.rename's guard, breaking
  the name-keyed presentation lookup

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/admin.py

@@ -156,6 +156,10 @@ async def tag_delete(tag_id: int):
             )
         except LookupError:
             return _bad("not_found", status=404)
+        except ValueError as exc:
+            # System tags (#128) — the training-hygiene machinery keys on
+            # these rows.
+            return _bad("system_tag", detail=str(exc))
     return jsonify(result)