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:

backend/app/services/tag_service.py

@@ -84,7 +84,11 @@ class TagService:
         - if fandom_id is set, the referenced tag must exist and have
           kind == TagKind.fandom
         """
-        name = normalize_tag_name(name)
+        # NOTE: case is NOT normalized here — find_or_create is the shared path
+        # the ML tagger / allowlist also use, and Title-Casing the booru
+        # vocabulary breaks allowlist matching. Display-casing (Title Case) is
+        # applied at the user-entry layer (api/tags create_tag) only (#701).
+        name = name.strip()
         if not name:
             raise TagValidationError("Tag name cannot be empty")
 
@@ -259,14 +263,14 @@ class TagService:
         with an existing tag of the same (kind, fandom_id) — the API turns
         that into a 409 merge hint.
         """
-        new_name = normalize_tag_name(new_name)
+        new_name = new_name.strip()
         if not new_name:
             raise TagValidationError("Tag name cannot be empty")
         tag = await self.session.get(Tag, tag_id)
         if tag is None:
             raise TagValidationError(f"Tag {tag_id} not found")
 
-        # Case-insensitive clash (#701) — colliding with a differently-cased tag
+        # Case-insensitive clash (#701) — renaming onto a differently-cased tag
         # is still a merge, not a silent fork.
         clash_stmt = (
             select(Tag)