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/utils/tag_names.py

@@ -0,0 +1,49 @@
+"""Normalization helpers for Tag display names.
+
+Keeps the first letter of every whitespace-separated word upper-case and
+preserves internal punctuation. Called from every character/fandom write
+path so typos like "misty" don't accumulate alongside "Misty".
+"""
+
+
+def underscores_to_spaces(s: str) -> str:
+    """Replace every underscore with a space.
+
+    Used by general/null-kind write paths and as a pre-step inside
+    normalize_display_name so WD14's 'misty_ketchum' and a hand-typed
+    'Misty Ketchum' collapse onto the same tag name.
+
+    Examples:
+        "big_hair"        -> "big hair"
+        "long_blue_hair"  -> "long blue hair"
+        ""                -> ""
+    """
+    return s.replace('_', ' ') if s else s
+
+
+def normalize_display_name(s: str) -> str:
+    """Title-case each whitespace-separated word. Preserves internal punctuation.
+
+    Underscores are converted to spaces first so WD14-style names normalize
+    identically to hand-typed names.
+
+    Examples:
+        "misty"           -> "Misty"
+        "misty ketchum"   -> "Misty Ketchum"
+        "misty_ketchum"   -> "Misty Ketchum"
+        "o'brien"         -> "O'brien"
+        "mary-kate"       -> "Mary-kate"
+        ""                -> ""
+        "   "             -> "   " (only whitespace — returned unchanged)
+    """
+    if not s:
+        return s
+    s = underscores_to_spaces(s)
+    parts = s.split(' ')
+    out: list[str] = []
+    for p in parts:
+        if p == '':
+            out.append(p)
+        else:
+            out.append(p[:1].upper() + p[1:])
+    return ' '.join(out)