fix: atomic burn-after-read claim (#58)

SoftDelete now reports whether it performed the delete (conditional
UPDATE ... WHERE deleted_at IS NULL checked via RowsAffected).
RegisterRead returns an admitted flag: legacy burn pastes admit exactly
one reader (the atomic soft-delete winner), and burn-after-N pastes
increment reads_used via a conditional UPDATE guarded on
reads_used < reads_limit, so concurrent readers cannot both consume the
final read. API, HTML, and raw read paths return 404 when the reader
loses the burn claim; content is never served twice.

OpenStore pins the SQLite pool to one connection: concurrent writes on
separate pooled connections surfaced SQLITE_BUSY as spurious 500s
instead of serializing.

Adds concurrency regression tests: 24 parallel readers of a burn paste
(exactly one receives content, none of the others leak it) and 30
parallel readers vs a 3-read budget (exactly 3 admitted, then 404).

internal/store/store.go

@@ -81,6 +81,11 @@ func OpenStore(path string) (*Store, error) {
 	if err != nil {
 		return nil, err
 	}
+	// #58: a single write connection. SQLite allows only one writer at a
+	// time; with multiple pooled connections concurrent writes surface as
+	// SQLITE_BUSY errors ("database is locked") instead of serializing, and
+	// the burn-after-read race tests saw spurious 500s under parallel reads.
+	db.SetMaxOpenConns(1)
 	s := &Store{db: db}
 	if err := s.migrate(); err != nil {
 		return nil, err
@@ -320,9 +325,17 @@ func (s *Store) MineOwner(id string) (string, error) {
 	return vid.String, nil
 }
 
-func (s *Store) SoftDelete(id string) error {
-	_, err := s.db.Exec(`UPDATE pastes SET deleted_at=? WHERE id=? AND deleted_at IS NULL`, time.Now().Unix(), id)
-	return err
+// SoftDelete marks a paste deleted (burned) atomically (#58): the deleted_at
+// IS NULL guard means only the first caller flips the row. Returns true when
+// this call performed the delete (RowsAffected > 0), false when the paste was
+// already deleted - callers use this to decide read admission atomically.
+func (s *Store) SoftDelete(id string) (bool, error) {
+	res, err := s.db.Exec(`UPDATE pastes SET deleted_at=? WHERE id=? AND deleted_at IS NULL`, time.Now().Unix(), id)
+	if err != nil {
+		return false, err
+	}
+	n, err := res.RowsAffected()
+	return n > 0, err
 }
 
 func (s *Store) IncrementViews(id string) {