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/api/server.go

@@ -29,11 +29,11 @@ type Config struct {
 }
 
 type apiServer struct {
-	store       *store.Store
-	cfg         Config
-	ui          *web.UI
-	settings    *settingsStore
-	adminKey    string
+	store    *store.Store
+	cfg      Config
+	ui       *web.UI
+	settings *settingsStore
+	adminKey string
 }
 
 func NewServer(st *store.Store, cfg Config, ui *web.UI, ss *settingsStore, adminKey string) *apiServer {
@@ -216,7 +216,12 @@ func (a *apiServer) handleGetPaste(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	}
-	rem, _ := a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow()) // #49 (also covers legacy burn)
+	// #58: only the reader that wins the atomic burn claim may see content.
+	rem, admitted := a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow())
+	if !admitted {
+		writeErr(w, 404, "paste not found")
+		return
+	}
 	writeJSON(w, 200, map[string]any{
 		"id": row.ID, "content": row.Content, "content_type": row.ContentType,
 		"language": store.NullStrPtr(row.Language), "title": store.NullStrPtr(row.Title), "created_at": row.CreatedAt,
@@ -240,7 +245,7 @@ func (a *apiServer) handleDeletePaste(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, 403, "not your paste")
 		return
 	}
-	if err := a.store.SoftDelete(row.ID); err != nil {
+	if _, err := a.store.SoftDelete(row.ID); err != nil {
 		writeErr(w, 500, "db error")
 		return
 	}
@@ -319,8 +324,13 @@ func (a *apiServer) handleRaw(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	// #49 decision: raw reads count against the read budget too, with the
-	// same per-viewer dedupe window as page views.
-	a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow())
+	// same per-viewer dedupe window as page views. #58: a reader that loses
+	// the burn claim must not receive the content.
+	_, admitted := a.store.RegisterRead(row, currentViewerID(r), a.burnViewerWindow())
+	if !admitted {
+		http.Error(w, "not found", 404)
+		return
+	}
 	// #34: content_type is attacker-controlled via the create API. Serving it
 	// verbatim let a paste be stored with text/html (or image/svg+xml) and
 	// render as active content on this origin when fetched from /raw —