#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/store/store.go

@@ -78,7 +78,8 @@ type CanRow struct {
 }
 
 type Store struct {
-	db *sql.DB
+	db   *sql.DB
+	blob BlobStore // #38: attachment byte storage (nil in some unit tests)
 }
 
 func OpenStore(path string) (*Store, error) {
@@ -95,6 +96,12 @@ func OpenStore(path string) (*Store, error) {
 	if err := s.migrate(); err != nil {
 		return nil, err
 	}
+	// #38: attachment blobs live beside the database under <db>.files
+	blobs, err := NewFsBlobStore(path + ".files")
+	if err != nil {
+		return nil, err
+	}
+	s.blob = blobs
 	return s, nil
 }
 
@@ -142,9 +149,26 @@ deletion_token TEXT
 		last_viewed INTEGER NOT NULL,
 		PRIMARY KEY (paste_id, viewer_id)
 	)`) // #49: per-viewer read dedupe window
+	// #38: file attachments, one per paste in iteration 1. mime is sniffed
+	// server-side before insert; bytes live in the BlobStore keyed
+	// <paste_id>/<sha256>.
+	s.db.Exec(`CREATE TABLE IF NOT EXISTS attachments (
+		id         TEXT PRIMARY KEY,
+		paste_id   TEXT NOT NULL,
+		filename   TEXT NOT NULL,
+		mime       TEXT NOT NULL,
+		size       INTEGER NOT NULL,
+		sha256     TEXT NOT NULL,
+		created_at INTEGER NOT NULL
+	)`)
+	s.db.Exec(`CREATE INDEX IF NOT EXISTS idx_attachments_paste ON attachments(paste_id)`)
 	return err
 }
 
+// Blobs returns the attachment blob store (nil when unavailable, e.g. some
+// unit-test stores).
+func (s *Store) Blobs() BlobStore { return s.blob }
+
 // SlugAlphabet is the paste-id charset (no ambiguous chars).
 var SlugAlphabet = "23456789abcdefghjkmnpqrstuvwxyz"