feat(tag-prefix): parse_kind_prefix util — IR-style \kind:name\ parser at the input boundary; KNOWN_KINDS = artist/character/fandom/series/meta/rating (excludes default \general\ and system-managed archive/post)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/utils/tag_prefix.py

@@ -0,0 +1,43 @@
+"""Parse the user-facing `kind:name` shortcut used by the add-tag input.
+
+Mirrors IR's app/utils/tag_prefix.py. Tag.name in FC is stored bare;
+the `kind:` prefix only exists as an input convention at user-facing
+places (image-modal add-tag input, future bulk-add forms). The parser
+is the single owner of the kind-string list — anything not in
+KNOWN_KINDS keeps its colon as literal text.
+"""
+
+from __future__ import annotations
+
+# Kinds the user can type as a prefix at the input boundary.
+# `general` is intentionally excluded — it's the default for un-prefixed
+# input. `archive` and `post` are system-managed and never user-typed.
+KNOWN_KINDS: frozenset[str] = frozenset({
+    "artist",
+    "character",
+    "fandom",
+    "series",
+    "meta",
+    "rating",
+})
+
+
+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 lowercase canonical and in
+    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("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.lower() in KNOWN_KINDS:
+            return prefix.lower(), rest.strip()
+    return None, raw.strip()