feat(ml): normalize Camie suggestion names to human-readable

Camie's booru-style vocab strings (`uchiha_sasuke_(naruto)`,
`#unicus_(idolmaster)`, `1000-nen_ikiteru_(vocaloid)`, `:/`) were
surfacing raw in SuggestionsPanel — and worse, the SAME raw string was
written to tag.name on Accept, polluting the DB with `underscored_lowercase`
names that don't match the operator's "Title Case" tag convention.

Add backend/app/services/ml/tag_name.py with a single normalize()
applying nine rules (strip leading junk #/./+/;/~/_/ws, drop trailing
_(disambiguator) blocks iteratively, strip wrapping quotes, underscores
to spaces, space after colon, title-case each word's first char,
preserve hyphens/apostrophes/digits, drop entries with no letters).

Wire into SuggestionService.for_image:
- raw Camie key kept for alias_map lookup (alias rows are hand-curated
  against raw keys; don't disturb)
- display_name = normalize(raw); None means drop the candidate
- existing-tag lookup widened to case-insensitive match against BOTH
  raw and normalized forms so legacy underscore-named Tag rows accepted
  before this change still surface as "existing" not "+ new"

tests/test_ml_suggestions.py

@@ -39,8 +39,9 @@ async def test_threshold_filters_low_confidence_general(db):
     await db.flush()
     sl = await SuggestionService(db).for_image(img.id)
     names = [s.display_name for s in sl.by_category.get("general", [])]
-    assert "sword" in names
-    assert "lowconf" not in names
+    # display_name is normalized (tag_name.normalize) before surfacing.
+    assert "Sword" in names
+    assert "Lowconf" not in names
 
 
 @pytest.mark.asyncio
@@ -84,7 +85,9 @@ async def test_raw_tag_creates_new(db):
     await db.flush()
     sl = await SuggestionService(db).for_image(img.id)
     chars = sl.by_category["character"]
-    assert chars[0].display_name == "brand_new_tag"
+    # display_name is the normalized Camie name (underscores -> spaces,
+    # title-cased), not the raw vocab key.
+    assert chars[0].display_name == "Brand New Tag"
     assert chars[0].creates_new_tag is True
     assert chars[0].canonical_tag_id is None