fix: chunk large chapter text for Kokoro TTS to prevent EOF on big inputs

Split chapter text into ~1000-char sentence-boundary chunks before sending
to kokoro-fastapi. Each chunk is generated individually and the raw MP3 bytes
are concatenated. This prevents the EOF / timeout failures that occur when
the server receives a very large single request (e.g. a full PDF 'Full Text'
chapter). chunkText() breaks at sentence endings (. ! ? newlines) to preserve
natural speech flow.

backend/internal/runner/helpers.go

@@ -19,3 +19,53 @@ func stripMarkdown(src string) string {
 	src = regexp.MustCompile(`\n{3,}`).ReplaceAllString(src, "\n\n")
 	return strings.TrimSpace(src)
 }
+
+// chunkText splits text into chunks of at most maxChars characters, breaking
+// at sentence boundaries (". ", "! ", "? ", "\n") so that the TTS service
+// receives natural prose fragments rather than mid-sentence cuts.
+//
+// If a single sentence exceeds maxChars it is included as its own chunk —
+// never silently truncated.
+func chunkText(text string, maxChars int) []string {
+	if len(text) <= maxChars {
+		return []string{text}
+	}
+
+	// Sentence-boundary delimiters — we split AFTER these sequences.
+	// Order matters: longer sequences first.
+	delimiters := []string{".\n", "!\n", "?\n", ". ", "! ", "? ", "\n\n", "\n"}
+
+	var chunks []string
+	remaining := text
+
+	for len(remaining) > 0 {
+		if len(remaining) <= maxChars {
+			chunks = append(chunks, strings.TrimSpace(remaining))
+			break
+		}
+
+		// Find the last sentence boundary within the maxChars window.
+		window := remaining[:maxChars]
+		cutAt := -1
+		for _, delim := range delimiters {
+			idx := strings.LastIndex(window, delim)
+			if idx > 0 && idx+len(delim) > cutAt {
+				cutAt = idx + len(delim)
+			}
+		}
+
+		if cutAt <= 0 {
+			// No boundary found — hard-break at maxChars to avoid infinite loop.
+			cutAt = maxChars
+		}
+
+		chunk := strings.TrimSpace(remaining[:cutAt])
+		if chunk != "" {
+			chunks = append(chunks, chunk)
+		}
+		remaining = strings.TrimSpace(remaining[cutAt:])
+	}
+
+	return chunks
+}
+