file-drop: temporary R2 relay

PUT /?name=notes.txt&ttl=3600
  Authorization: Bearer $DROP_TOKEN
  body is the file; Content-Length is required, max 100 MiB.
  ttl is seconds, default 1 day, minimum 1 second, max 7 days.

GET /:id            download (HEAD returns headers only)
GET /:id?meta=1     metadata only
POST /:id/share     JSON {"ttl":3600}, optional; max 7 days and bounded by file expiry
DELETE /:id         delete now

Authenticated share creation returns a signed GET/HEAD URL, usable without a bearer token.
Metadata, uploads, share creation, and deletion require the bearer token.
Expired objects are unavailable immediately, deleted on read, and swept hourly (UTC).