fix(tests): test_api_tags prefix tests use character: not artist: (KNOWN_KINDS dropped artist)

The two prefix-parsing tests were pinned to `artist:Eric`, but `artist`
was removed from KNOWN_KINDS in commit 4cad07a (provenance is a separate
axis from tags). The parser now keeps `artist:` literal, so the assertion
`body["name"] == "Eric"` failed.

Repointed to `character:Saber` (still in KNOWN_KINDS). Also updated the
stale `artist:` docstring example in parse_kind_prefix to `fandom:`.

Caught by [[reference-grep-pinned-tests-in-plans]] — should have grep'd
tests/ for `artist:` when shrinking KNOWN_KINDS. Banking the miss.

backend/app/utils/tag_prefix.py

@@ -37,7 +37,7 @@ def parse_kind_prefix(raw: str) -> tuple[str | None, str]:
         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")
+        parse_kind_prefix("fandom: FSN ")    -> ("fandom", "FSN")
     """
     if ":" in raw:
         prefix, rest = raw.split(":", 1)