feat: ML tag suggestions, character/fandom integrity, underscores, modal polish

Consolidated merge of feat/tag-suggestions branch. Original 64-commit history
was lost to git-object corruption in a Nextcloud-synced checkout; this single
commit captures the equivalent diff.

Includes:
- pgvector-backed tag suggestion infra (WD14 + SigLIP centroids, ml-worker
  container, Celery tasks, suggestion service, accept/reject endpoints + modal
  UI with green/red chip buttons)
- Character/fandom integrity: title-case normalization on every write path,
  fandom-id backfill, maintenance task + settings button, migrations g26041901
  + h26041901 to canonicalize legacy rows with case-only duplicate merging
- Tag-underscores + modal polish: WD14 name canonicalization at emit + accept
  + add/bulk-add paths, migration i26041901 for legacy-row rename-or-merge
  across character/fandom/NULL kinds, suggestion-accept refresh parity via
  awaited loadTags, persistent chip tint

app/tasks/maintenance.py

@@ -0,0 +1,66 @@
+"""Maintenance-queue Celery tasks. Lightweight, user-triggered, non-ML."""
+import logging
+from celery import shared_task
+from sqlalchemy import text
+
+from app import db
+from app.models import Tag
+
+log = logging.getLogger(__name__)
+
+
+@shared_task(
+    name='app.tasks.maintenance.sync_character_fandoms',
+    soft_time_limit=120,
+    time_limit=180,
+)
+def sync_character_fandoms():
+    """For every character tag with a non-null fandom_id, attach the fandom
+    tag to every image tagged with the character but not the fandom.
+
+    Additive only — never removes fandom tags.
+
+    Returns a summary dict with counts.
+    """
+    chars = (
+        Tag.query
+        .filter_by(kind='character')
+        .filter(Tag.fandom_id.isnot(None))
+        .all()
+    )
+    total_added = 0
+    failed = 0
+    for char in chars:
+        try:
+            result = db.session.execute(
+                text("""
+                    INSERT INTO image_tags (image_id, tag_id)
+                    SELECT it.image_id, :fandom_id
+                    FROM image_tags it
+                    WHERE it.tag_id = :char_id
+                      AND NOT EXISTS (
+                          SELECT 1 FROM image_tags it2
+                          WHERE it2.image_id = it.image_id
+                            AND it2.tag_id = :fandom_id
+                      )
+                """),
+                {'char_id': char.id, 'fandom_id': char.fandom_id},
+            )
+            total_added += result.rowcount or 0
+            db.session.commit()
+        except Exception as e:
+            db.session.rollback()
+            failed += 1
+            log.warning(
+                "sync_character_fandoms: char tag %s (%s) failed: %s",
+                char.id, char.name, e,
+            )
+    log.info(
+        "sync_character_fandoms: scanned %d characters, added %d image↔fandom links, %d failed",
+        len(chars), total_added, failed,
+    )
+    return {
+        'characters_scanned': len(chars),
+        'links_added': total_added,
+        'characters_failed': failed,
+    }