fix(i18n): hold every refusal reason by type, and refuse what the scan cannot read

Review round 1 of the refusal contract: it claimed more than it held.

- The other reason carriers get their Literal too (BlockedUrl, ClipError,
  FolderError, the playlist and sign-in refusal helpers), so the type
  checker refuses a reason passed by variable or an unknown literal, and
  the contract reads one shape.
- A literal reason in a detail dict built anywhere counts as a reason of
  its code; a code the scan cannot read, a refused() whose reason it
  cannot read, and a new reason passed by variable each fail the contract
  until listed with what holds them.
- A sign-in refusal key the backend no longer sends is caught too.
- localizeDetail's numbers (download cap in GB, upload cap in bytes) and
  the shared-sentence fallbacks are tested in both languages.
- One Literal parser in the contract; a stale comment fixed.

backend/app/downloads/clip.py

@@ -22,6 +22,7 @@ import json
 import math
 import re
 from pathlib import Path
+from typing import Literal
 
 from app.downloads.edit import AAC_ARGS, EVEN_FRAME, INPUT_GUARD, X264_ARGS
 from app.plex.sync import HDR_TRANSFERS
@@ -57,11 +58,26 @@ _CROP_ASPECTS = (0.25, 4.0)
 _EDGE_SLACK = 1e-3
 
 
+# Every reason a clip is refused; the UI names each one (`clip.errors.<reason>`, held by the frontend's
+# backendIds contract, which reads this Literal).
+ClipReason = Literal[
+    "bad_source",
+    "bad_range",
+    "too_short",
+    "too_long",
+    "bad_audio",
+    "bad_crop",
+    "not_found",
+    "media_missing",
+    "past_end",
+]
+
+
 class ClipError(Exception):
     """A clip request that cannot be cut; `reason` is an i18n-mappable key, `params` the numbers its
     message names (the cap itself, so the client never says a stale one)."""
 
-    def __init__(self, reason: str, **params: float):
+    def __init__(self, reason: ClipReason, **params: float):
         self.reason = reason
         self.params = params
         super().__init__(reason)