fix(tags): Title-Case operator-entered tags at create endpoint only (plan #701)

normalize_tag_name (per-word capitalize + whitespace collapse) is applied in
the POST /api/tags handler so operator-entered tags get clean display casing.
It is NOT applied in the shared find_or_create / rename paths — those are used
by the ML tagger and allowlist matching, which must preserve the booru
vocabulary's original casing (Title-Casing it broke apply_allowlist matching).

find_or_create / rename keep case-insensitive lookup + clash detection so a
differently-cased entry dedups onto the existing tag instead of forking.

Tests updated to expect Title-Cased create output (sunset→Sunset,
character:Saber→Character:saber, http://example.com→Http://example.com) and a
dedicated normalize_tag_name unit test.

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

backend/app/api/tags.py

@@ -14,6 +14,7 @@ from ..services.tag_service import (
     TagMergeConflict,
     TagService,
     TagValidationError,
+    normalize_tag_name,
 )
 from ..utils.tag_prefix import parse_kind_prefix
 
@@ -141,6 +142,11 @@ async def create_tag():
 
     fandom_id = body.get("fandom_id")
 
+    # #701: Title-Case operator-entered tags. Only here (the explicit create
+    # endpoint), NOT in the shared find_or_create — the ML tagger uses that path
+    # and must keep the booru vocabulary's casing for allowlist matching.
+    name = normalize_tag_name(name)
+
     async with get_session() as session:
         svc = TagService(session)
         try: