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/feature-flags.ts

@@ -77,20 +77,31 @@ export function isDirectFileUploadEnabled() {
   return isS3VideoUploadsEnabled() || isBunnyUploadsEnabled();
 }
 
-export function getMaxVideoUploadBytes(): bigint {
+/**
+ * The per-file ceiling for a host that has no billing, and therefore no per
+ * account quota to derive one from.
+ */
+export const DEFAULT_MAX_VIDEO_UPLOAD_BYTES =
+  BigInt(5) * BigInt(1024) * BigInt(1024) * BigInt(1024);
+
+/**
+ * A flat per-file ceiling this host has pinned, or null when it has not.
+ *
+ * Null is the ordinary case. The ceiling that applies to a paying account is a
+ * share of that account's own storage quota, which one number in the
+ * environment cannot express (see `getMaxVideoUploadBytesForUser`). A host that
+ * wants an absolute cap on top of that still sets this, and the lower of the
+ * two wins.
+ */
+export function getConfiguredMaxVideoUploadBytes(): bigint | null {
   const raw = process.env.OPENFRAME_MAX_VIDEO_UPLOAD_BYTES?.trim();
-  if (!raw) {
-    return BigInt(5) * BigInt(1024) * BigInt(1024) * BigInt(1024);
-  }
+  if (!raw) return null;
 
   try {
     const parsed = BigInt(raw);
-    if (parsed <= BigInt(0)) {
-      return BigInt(5) * BigInt(1024) * BigInt(1024) * BigInt(1024);
-    }
-    return parsed;
+    return parsed > BigInt(0) ? parsed : null;
   } catch {
-    return BigInt(5) * BigInt(1024) * BigInt(1024) * BigInt(1024);
+    return null;
   }
 }