fix(kb/ingest): merge blocks on the same row so broken-word pages read

The previous block-based parser left each pymupdf block as its own line.
That's fine for prose but wrong for ספר הליקויים's cover and preface,
where each word is its own block — you got "ו אנ / מתכבד / ים" instead
of "ואנו מתכבדים". Merge blocks whose y-coordinates are within 4 pixels
into a single line, right-to-left, joined by spaces.

Content pages (where blocks already contain whole paragraphs) are
unaffected because a single block on one row stays as one row.

Refs Task Master #2

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/services/kb/ingest.py

@@ -38,32 +38,59 @@ def _coerce_date(value) -> _dt.date | None:
 
 
 def _parse_pdf(data: bytes) -> str:
-    """Extract text from a PDF, preferring block-based parsing.
+    """Extract text from a PDF, preferring block-based parsing with row merging.
 
-    Plain `get_text("text")` reads runs in raw glyph order which interleaves
-    column labels, page numbers, and body copy for multi-column Hebrew PDFs
-    (e.g. ספר הליקויים). Using `get_text("blocks")` and sorting by vertical
-    position then horizontal position keeps each paragraph intact and
-    preserves reading order top-to-bottom.
+    Plain `get_text("text")` interleaves columns and floating labels for
+    multi-column Hebrew PDFs (e.g. ספר הליקויים) — words come out split to
+    single letters on separate lines. Using `get_text("blocks")` and sorting
+    by vertical position keeps paragraphs intact for well-formatted pages.
+
+    Some PDFs (ספר הליקויים's title/preface pages) emit each word as its
+    own block. Sorting blocks by y-position alone leaves them one-word-per-
+    line. To fix that, we merge blocks that share the same visual row
+    (|Δy| ≤ row_tolerance) into a single line joined by spaces, right-to-
+    left. That makes prose pages readable without affecting well-formed
+    content pages.
     """
     parts: list[str] = []
+    row_tolerance = 4.0  # pixels; blocks within this y-range are the same line
+
     with fitz.open(stream=data, filetype="pdf") as doc:
         for page in doc:
-            blocks = page.get_text("blocks") or []
-            # Filter out empty / image-only blocks and sort by (y, x). For RTL
-            # languages, sorting by descending x within the same y keeps the
-            # reading order correct (right-to-left); pymupdf's default block
-            # order already respects this but only per-column — sorting
-            # explicitly makes it robust across oddly-placed floaters.
+            raw = page.get_text("blocks") or []
             textual = [
-                (b[1], -b[0], b[4])
-                for b in blocks
+                (float(b[0]), float(b[1]), float(b[2]), float(b[3]), b[4])
+                for b in raw
                 if isinstance(b, tuple) and len(b) >= 5 and isinstance(b[4], str) and b[4].strip()
             ]
-            textual.sort()  # by y ascending, then x descending (via negation)
-            # Join paragraphs with a blank line so the chunker's header regexes
-            # still see line boundaries.
-            parts.append("\n\n".join(b[2].rstrip() for b in textual))
+            if not textual:
+                continue
+
+            # Sort by y (top→bottom), then by -x so RTL rows read right→left
+            textual.sort(key=lambda t: (t[1], -t[0]))
+
+            # Group into rows by y proximity, then join each row with a space.
+            lines: list[str] = []
+            current_row: list[tuple[float, str]] = []
+            current_y: float | None = None
+            for x0, y0, x1, y1, text in textual:
+                text = text.strip()
+                if not text:
+                    continue
+                if current_y is None or abs(y0 - current_y) <= row_tolerance:
+                    current_row.append((x0, text))
+                    current_y = y0 if current_y is None else current_y
+                else:
+                    # Flush previous row
+                    current_row.sort(key=lambda p: -p[0])  # RTL
+                    lines.append(" ".join(p[1] for p in current_row))
+                    current_row = [(x0, text)]
+                    current_y = y0
+            if current_row:
+                current_row.sort(key=lambda p: -p[0])
+                lines.append(" ".join(p[1] for p in current_row))
+
+            parts.append("\n\n".join(lines))
     return "\n\n".join(p for p in parts if p).strip()