feat(utils): add parse_kind_prefix helper

Owns the 'character:Saber' user-input shortcut parser in one place.
Invoked by add_tag, bulk_add_tag, and the migration's sanity check.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/utils/tag_prefix.py

@@ -0,0 +1,43 @@
+"""Parse the user-facing `kind:name` shortcut used by the add-tag input.
+
+After the bare-name refactor, `Tag.name` no longer stores the `kind:` prefix.
+The prefix lives only as an input convention in a handful of user-facing
+places (modal add-tag input, bulk-add-tag form, import paths that receive
+user-typed strings). This module owns that parser.
+"""
+from __future__ import annotations
+
+# The set of tag kinds the app recognizes as prefixable at the input boundary.
+# `user` is intentionally excluded — user-typed general tags never carry a
+# prefix. Strings like "http://example" keep their colon as literal text
+# because "http" is not in this set.
+KNOWN_KINDS: frozenset[str] = frozenset({
+    'character',
+    'fandom',
+    'artist',
+    'series',
+    'post',
+    'archive',
+    'meta',
+})
+
+
+def parse_kind_prefix(raw: str) -> tuple[str | None, str]:
+    """Split a raw user-typed tag string into (kind, name).
+
+    Returns:
+        (kind, name) where kind is one of KNOWN_KINDS, or
+        (None, raw.strip()) if no recognized prefix is present.
+        `name` is always whitespace-stripped.
+
+    Examples:
+        parse_kind_prefix('character:Saber')  -> ('character', 'Saber')
+        parse_kind_prefix('sunset')           -> (None, 'sunset')
+        parse_kind_prefix('http://example')   -> (None, 'http://example')
+        parse_kind_prefix('artist: Eric ')    -> ('artist', 'Eric')
+    """
+    if ':' in raw:
+        prefix, rest = raw.split(':', 1)
+        if prefix in KNOWN_KINDS:
+            return prefix, rest.strip()
+    return None, raw.strip()