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
 

tests/test_ml_tag_name.py

@@ -0,0 +1,53 @@
+import pytest
+
+from backend.app.services.ml.tag_name import normalize
+
+
+@pytest.mark.parametrize(
+    "raw, expected",
+    [
+        # Rule 4: underscores -> spaces; rule 7: title case
+        ("light_purple_hair", "Light Purple Hair"),
+        ("no_pants", "No Pants"),
+        ("year_2005", "Year 2005"),
+        # Single-word still title-cased
+        ("sword", "Sword"),
+        # Rule 3: drop trailing _(disambiguator)
+        ("uchiha_sasuke_(naruto)", "Uchiha Sasuke"),
+        ("apple_(fruit)", "Apple"),
+        ("kirby_(series)", "Kirby"),
+        # Repeated trailing disambig blocks
+        ("foo_(bar)_(baz)", "Foo"),
+        # Rule 1: leading junk chars
+        ("#unicus_(idolmaster)", "Unicus"),
+        (".52_gal_(splatoon)", "52 Gal"),
+        ("+_+_smile_(emote)", "Smile"),
+        # Rule 5: space after colon
+        ("nier:automata", "Nier: Automata"),
+        # Already-spaced colon left alone
+        ("nier: automata", "Nier: Automata"),
+        # Rule 6: hyphens preserved
+        ("1000-nen_ikiteru_(vocaloid)", "1000-nen Ikiteru"),
+        ("well-known_face", "Well-known Face"),
+        # Rule 2: wrapping quotes
+        ('"pile_em_up"_(genshin_impact)', "Pile Em Up"),
+        ("'foo_bar'", "Foo Bar"),
+        # Rule 8: emoticons -> None
+        (":/", None),
+        (";)", None),
+        ("+_+", None),
+        ("^_^", None),
+        # Empty / whitespace-only
+        ("", None),
+        ("   ", None),
+        ("___", None),
+        # Apostrophe inside word — preserved, not title-cased
+        ("it's_okay", "It's Okay"),
+        # Digit-only still surfaces (year tags)
+        ("2005", "2005"),
+        # Multi-space collapse
+        ("foo___bar", "Foo Bar"),
+    ],
+)
+def test_normalize(raw, expected):
+    assert normalize(raw) == expected