fix(aliases): store modal alias under raw model key + make aliases visible/manageable

The headline bug: aliases created from the modal NEVER resolved. Create
sent the normalized display name ('Sword', 'Uchiha Sasuke') while
resolution keys on the raw booru model key ('sword', 'uchiha_sasuke',
case-sensitive) — so the mapping was stored under a key nothing looks up,
and the prediction kept reappearing unaliased. The raw key wasn't even in
the /suggestions response, so the modal couldn't send it.

- Suggestion now carries raw_name (the model key an alias must use) and
  via_alias (surfaced via an operator alias); both serialized by the API.
- Modal alias-create sends raw_name, not display_name (the fix). Aliased
  suggestions show an 'alias' badge and a 'Remove alias' action; 'Treat as
  alias for…' is hidden for centroid hits (no model key) and already-aliased
  rows.
- Tag-side management: TagCard ⋮ → 'Aliases…' opens a dialog listing the
  model keys that fold into a tag, with remove (GET /api/tags/<id>/aliases +
  AliasService.list_for_tag). Creation stays in the modal suggestion flow.

Tests: full API round-trip locking the raw-key contract (raw_name exposed →
alias authored with it → resolves + via_alias on a later image);
list_for_tag (service + API); via_alias/raw_name on the existing service
suggestion tests. No migration.

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

backend/app/services/ml/suggestions.py

@@ -31,6 +31,14 @@ class Suggestion:
     score: float
     source: str  # 'tagger' | 'centroid' | 'both'
     creates_new_tag: bool
+    # raw_name = the booru model vocab key behind this suggestion. It's the key
+    # an alias MUST be stored under (resolution looks up the raw key), so the
+    # modal needs it to author an alias correctly. None for centroid-only hits
+    # (no underlying prediction → nothing to alias).
+    raw_name: str | None = None
+    # via_alias = this suggestion was surfaced because an operator alias remapped
+    # the raw prediction to this canonical tag. Lets the UI mark it + offer undo.
+    via_alias: bool = False
 
 
 @dataclass
@@ -161,6 +169,11 @@ class SuggestionService:
                     if existing.source != sug.source
                     else existing.source,
                     creates_new_tag=existing.creates_new_tag,
+                    # Keep the alias identity from `existing`: the tagger pass
+                    # (which carries raw_name / via_alias) runs before centroid
+                    # augmentation, so it's always the first writer for a key.
+                    raw_name=existing.raw_name,
+                    via_alias=existing.via_alias,
                 )
 
         for raw, display, category, conf in candidates:
@@ -177,6 +190,8 @@ class SuggestionService:
                         score=conf,
                         source="tagger",
                         creates_new_tag=False,
+                        raw_name=raw,
+                        via_alias=True,
                     ),
                 )
             else:
@@ -208,6 +223,8 @@ class SuggestionService:
                             score=conf,
                             source="tagger",
                             creates_new_tag=False,
+                            raw_name=raw,
+                            via_alias=False,
                         ),
                     )
                 else:
@@ -220,6 +237,8 @@ class SuggestionService:
                             score=conf,
                             source="tagger",
                             creates_new_tag=True,
+                            raw_name=raw,
+                            via_alias=False,
                         ),
                     )