feat(uploads): size one upload against the account's own quota

The per-file ceiling was a flat 5 GiB from the environment, which is both
too small for a paying account with 200 GB of storage and unaware of what
an upload actually costs. The provider derives its own renditions from the
file (1080p, 720p and down) and bills them to the same account, so a file
allowed to fill the quota exactly is over it by the time it finishes
processing.

The ceiling is now 80% of whatever limit the account is held to: 160 GB on
the plan, 2.4 GB on a cardless trial, and it moves on its own when either
number changes. OPENFRAME_MAX_VIDEO_UPLOAD_BYTES keeps working as an
absolute cap for a host that wants one, where the lower of the two applies,
and an instance running without billing has no quota to divide and falls
back to the flat 5 GiB. The refusal now names the ceiling, which the old
one left the client to guess.

Finalize re-checks only the host cap. Re-deriving the account's ceiling
there would delete a finished upload over a plan that lapsed while the
bytes were in flight, and an upload larger than what was declared is
already caught by the declared-size check beside it.

lib/upload-size.ts

@@ -6,9 +6,28 @@
 
 export type DeclaredUploadSize = { sizeBytes: bigint } | { error: string };
 
+/**
+ * A byte count in the unit a person reading a limit expects.
+ *
+ * Whole gigabytes where the number is whole, which most ceilings we ship are,
+ * and one decimal otherwise so a share of a small quota is not rounded into a
+ * lie. Lives here rather than next to the quota because the upload routes need
+ * it too, and this module imports nothing.
+ */
+export function formatSizeLimit(bytes: bigint): string {
+  const gigabytes = Number(bytes) / 1024 ** 3;
+  if (gigabytes < 1) return `${Math.round(Number(bytes) / 1024 ** 2)} MB`;
+  return `${Number.isInteger(gigabytes) ? gigabytes : gigabytes.toFixed(1)} GB`;
+}
+
+/** The refusal, with the ceiling in it, so the client knows what would fit. */
+export function uploadTooLargeMessage(maxBytes: bigint): string {
+  return `File exceeds the maximum allowed upload size (${formatSizeLimit(maxBytes)})`;
+}
+
 /**
  * Reads a declared upload size, refusing anything that is not a whole positive
- * number of bytes within the host's per-file ceiling.
+ * number of bytes within the ceiling this account is held to.
  *
  * The number is the client's word and is treated as such. Overstating it only
  * spends the caller's own quota, and understating it is caught where the bytes
@@ -34,7 +53,7 @@ export function parseDeclaredUploadSize(raw: unknown, maxBytes: bigint): Declare
   }
 
   if (sizeBytes > maxBytes) {
-    return { error: 'File exceeds the maximum allowed upload size' };
+    return { error: uploadTooLargeMessage(maxBytes) };
   }
 
   return { sizeBytes };