#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/server.go

@@ -113,7 +113,8 @@ func (a *apiServer) routes() http.Handler {
 	r.Get("/can/{id}", a.handleCanPage)
 	r.Post("/can/{id}", a.handleCanPage)
 
-	// raw
+	// raw + #38 attachment serving
+	r.Get("/f/{aid}/{filename}", a.handleServeAttachment)
 	r.Get("/raw/{id}", a.handleRaw)
 
 	// web pages
@@ -180,6 +181,11 @@ func (a *apiServer) handleCreatePaste(w http.ResponseWriter, r *http.Request) {
 		writeRateLimited(w, 1)
 		return
 	}
+	ct := r.Header.Get("Content-Type")
+	if strings.HasPrefix(ct, "multipart/form-data") {
+		a.handleCreatePasteMultipart(w, r, s) // #38: file attachment create
+		return
+	}
 	var p store.Paste
 	if err := json.NewDecoder(r.Body).Decode(&p); err != nil {
 		if isBodyTooLarge(err) { // #68: body cut off by MaxBytesReader