C1+C3 review findings: streaming decompression-bomb guard + stored-XSS stripping

C1 (decompression bomb): wire the shared common._copy_stream_with_cap into both
zip-extraction paths. app.py._safe_extract_zip now streams each EPUB member with
a WRITTEN-byte counter (app.py:456) instead of an unbounded copyfileobj, so a
member that inflates a small header into a huge stream is stopped mid-write.
pandoc_converter._validate_office_zip now streams every member's decompressed
bytes through the same guard (pandoc_converter.py:134) before pandoc reads them.

C3 (stored XSS): common.html_to_pdf_bytes now strips stored-XSS vectors via
common._strip_scripts (common.py:320) BEFORE WeasyPrint renders, so every
non-pandoc render path (html, svg, epub, md, csv, xml) neutralizes <script>/
<iframe> content, on* handlers and javascript:/vbscript: URLs. Regression tests
added to tests/test_review_findings_20260903.py (C1 unit+e2e, C3 unit+e2e).

converters/common.py

@@ -177,24 +177,23 @@ def _strip_scripts(html: str) -> str:
         r"\s+on\w+\s*=\s*(\"[^\"]*\"|'[^']*'|[^\s>]+)", "", html,
         flags=re.IGNORECASE,
     )
+
     # javascript:/vbscript: URLs in href/src -> blank the attribute.
+    # group(4) captures the value INCLUDING its surrounding quotes, so the
+    # scheme check strips the quotes first; otherwise the leading quote makes
+    # "javascript:" never match and the handler silently survives (the C3 bug).
     def _blank(match):
-        attr, quote, val = (
-            match.group(1), match.group(2), match.group(3)
+        quote, scheme = (
+            match.group(4)[0],
+            match.group(4).strip().strip('"').strip("'").strip(),
         )
-        stripped = val.strip()
-        if re.match(r"(?:javascript|vbscript):", stripped, re.IGNORECASE):
-            return f"{attr}={quote}{quote}"
+        if re.match(r"(?:javascript|vbscript):", scheme, re.IGNORECASE):
+            return f"{match.group(1)}{match.group(2)}{match.group(3)}{quote}{quote}"
         return match.group(0)
 
     html = re.sub(
         r"(\s)(href|src)(\s*=\s*)(\"[^\"]*\"|'[^']*'|[^\s>]+)",
-        lambda m: (
-            f'{m.group(1)}{m.group(2)}{m.group(3)}{m.group(4).strip()}'
-            if re.match(r"(?:javascript|vbscript):",
-                        m.group(4).strip(), re.IGNORECASE)
-            else m.group(0)
-        ),
+        _blank,
         html, flags=re.IGNORECASE,
     )
     return html
@@ -222,12 +221,17 @@ def zip_header_guard(infos, cap) -> None:
             )
 
 
-def _copy_stream_with_cap(src, dst, cap) -> None:
+def _copy_stream_with_cap(src, dst, cap) -> int:
     """Stream-copy src\u2192dst counting WRITTEN bytes; abort mid-stream if the
     decompressed total exceeds `cap` (C1 real enforcement).
 
-    Unlike a header-only check, this counts actual bytes delivered to disk,
-    so a zip that inflates a 5 MB header to 50 GB is stopped mid-write.
+    Unlike a header-only check, this counts actual bytes delivered to disk, so a
+    zip that inflates a 5 MB header to 50 GB is stopped mid-write. `dst` may be
+    None: when None the bytes are counted but not written (a pure pre-check that
+    must not touch disk), e.g. streaming the decompressed size of an office zip
+    before pandoc reads it. Always returns the total WRITTEN bytes; callers that
+    write to a file ignore the return.
+
     Satisfies: C1
     """
     chunk_size = 64 * 1024
@@ -235,7 +239,7 @@ def _copy_stream_with_cap(src, dst, cap) -> None:
     while True:
         chunk = src.read(chunk_size)
         if not chunk:
-            return
+            return written
         written += len(chunk)
         if written > cap:
             raise ValueError(
@@ -243,7 +247,9 @@ def _copy_stream_with_cap(src, dst, cap) -> None:
                 f"{written} bytes (cap {cap}); aborting extraction to "
                 "prevent disk-fill"
             )
-        dst.write(chunk)
+        if dst is not None:
+            dst.write(chunk)
+    return written
 
 
 # Resource-attribute names whose string value WeasyPrint may fetch from the
@@ -305,6 +311,13 @@ def html_to_pdf_bytes(html: str,
     ever runs, so every converter rendering user HTML/SVG is SSRF-safe by default.
     Satisfies: C2
     """
+    # C3: strip stored-XSS vectors BEFORE WeasyPrint renders the document.
+    # This is the single choke point every HTML/SVG/MD/CSV/XML/text converter
+    # flows through (svg renders here directly; the rest reach it via
+    # common.convert_single -> html_to_pdf_bytes), so one strip here covers the
+    # whole non-pandoc render surface. Harmless no-op for json/text, which escape
+    # their content into <...> so the script regex never matches.
+    html = _strip_scripts(html)
     scan_for_remote(html)
     from weasyprint import CSS, HTML