feat(comments): carry a batch of screenshots on one comment

A comment held one image, and the paste handler took the first item off the
clipboard and dropped the rest. Reviewing a cut usually means several
screenshots about the same moment, which meant one comment per screenshot or
one screenshot and a paragraph describing the others. Editing a comment could
not attach anything at all: the edit box had no paste handler, no file picker
and no way to remove what was already there.

A comment now carries up to five images, in the composer, in a reply and in
the editor. One paste stages every image on the clipboard, the file picker
takes a multiple selection, and a drop lands on whichever editor is open. Over
the cap the extras are refused out loud rather than dropped quietly. A single
image still fills the width; several tile into a grid, and either opens full
screen on click.

The images move into their own table. `comments.imageUrl` stays and follows the
first of them, so a reader that has not been updated keeps working, and the
migration copies the existing attachments across so the new table is complete
from the first read. Every path that resolves a URL back to a comment now asks
the new table: R2 cleanup, the orphan sweep, the storage accounting and the
reference checks that decide whether an object can be deleted. Left on the old
column they would have treated images two through five as unreferenced and
swept them.

Detaching an image while editing only breaks the link. The file stays in R2 and
in the assets pane, which is where it is deleted from and where its bytes are
already billed.

lib/upload-freshness.ts

@@ -0,0 +1,40 @@
+import { HeadObjectCommand } from '@aws-sdk/client-s3';
+import { r2Client, R2_BUCKET_NAME } from '@/lib/r2';
+
+/**
+ * How long an uploaded file may sit in R2 before the row that would claim it has
+ * to exist. Anything older is treated as an expired upload, so a URL cannot be
+ * replayed later to attach a file the caller no longer has a right to.
+ */
+export const UNATTACHED_UPLOAD_TTL_MS = 15 * 60 * 1000;
+
+export type AttachmentCheck = { isFresh: boolean; sizeBytes: bigint };
+
+/**
+ * Confirm an upload URL points at an object that was written just now, and
+ * report its size so the caller can bill it against a quota.
+ */
+export async function isFreshAttachment(
+  url: string,
+  kind: 'audio' | 'image'
+): Promise<AttachmentCheck> {
+  const prefix = kind === 'audio' ? '/api/upload/audio/' : '/api/upload/image/';
+  if (!url.startsWith(prefix)) return { isFresh: false, sizeBytes: BigInt(0) };
+
+  const filename = url.slice(prefix.length);
+  const key = kind === 'audio' ? `voice/${filename}` : `images/${filename}`;
+
+  try {
+    const head = await r2Client.send(
+      new HeadObjectCommand({
+        Bucket: R2_BUCKET_NAME,
+        Key: key,
+      })
+    );
+    if (!head.LastModified) return { isFresh: false, sizeBytes: BigInt(0) };
+    const isFresh = Date.now() - head.LastModified.getTime() <= UNATTACHED_UPLOAD_TTL_MS;
+    return { isFresh, sizeBytes: BigInt(head.ContentLength ?? 0) };
+  } catch {
+    return { isFresh: false, sizeBytes: BigInt(0) };
+  }
+}