#38 iteration 1: file attachments, 1 file per paste

- internal/store/blob.go: BlobStore interface + fs implementation with
  traversal-safe keys (<paste-id>/<sha256>), put/get/stat/delete
- attachments table migration (id, paste_id, filename sanitized to 255,
  mime sniffed server-side, size, sha256, created_at)
- POST /api/pastes now accepts multipart/form-data with a 'file' part;
  1 file = 1 paste: file replaces text content when both are sent
- 25 MB per-file limit enforced server-side (413 file_too_large)
- GET /f/{attachment-id}/{filename}: stored sniffed mime, nosniff,
  inline only for images/pdf, html/svg/xml forced to text/plain (#34 rule)
- paste view renders attachment chip + inline image preview
- /new: dropzone with file picker, drag-and-drop, Ctrl+V file paste,
  file chip with name/size/remove, matches pill/radius design
- tests: blob roundtrip/traversal/sanitize; multipart create (mime
  sniffing, client mime ignored, size limit, two-file reject, html/svg
  forcing, 404s, password/expiry fields)

internal/api/validate.go

@@ -18,6 +18,10 @@ import (
 func (a *apiServer) maxRequestBody() int64 {
 	s := a.settings.get()
 	max := s.MaxContentBytes + 4096
+	// #38: multipart create carries up to one 25 MB file plus overhead
+	if a.cfg.MaxItemBytes+1<<20 > max {
+		max = a.cfg.MaxItemBytes + 1<<20
+	}
 	if max < 64*1024 {
 		max = 64 * 1024
 	}