Implementace prevodu HTML na PDF

Sluzba prijme adresu HTML dokumentu nebo HTML v tele requestu a vrati PDF.
Navrzena pro dokumenty o stovkach az tisicich stranek.

Rendering:
- WeasyPrint jako vychozi engine, spravne CSS Paged Media, nizka pametova
  narocnost, bez JavaScriptu
- Chromium pres Playwright pro dokumenty dokreslovane skripty
- rezim auto s detekci skriptu a fallbackem pri selhani WeasyPrintu

Velke dokumenty:
- deleni na casti na strukturalnich hranicich, rez nikdy uvnitr tabulky
  nebo odstavce
- dvoupruchodovy render obsahu se skutecnymi cisly stranek, pozice nadpisu
  se ctou z kotev hlasenych u kazde stranky
- cislovani stranek bud pres CSS countery, nebo pres cislovaci vrstvu
  nastampovanou na hotove PDF, rozmer stranky se cte z vysledneho souboru
- Chromium se restartuje po N jobech, nikdy vsak behem beziciho renderu

API:
- POST /convert synchronne, POST /jobs asynchronne se sledovanim stavu,
  stahovanim vysledku, rusenim a volitelnym callbackem
- GET /health s overenim dostupnosti obou enginu a stavem fronty
- OpenAPI respektuje prefix reverse proxy pres root_path

Bezpecnost a provoz:
- SSRF kontrola po DNS resolvu, na kazdem presmerovani a u vsech pozadavku
  prohlizece
- nedostupne assety render nezastavi, ale hlasi se v odpovedi i v logu
- fronta s omezenym poctem workeru, rozpracovane joby se pri ukonceni
  oznaci jako failed, nezmizi potichu
- strukturovane JSON logovani s job_id
- vsechny limity vypnute ve vychozim stavu

Dockerfile je dvoufazovy, obsahuje zavislosti WeasyPrintu, Chromium
a fonty s ceskou diakritikou.

Autentizace zamerne neni implementovana, zpusob predavani neni domluveny.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/engines/weasy.py

@@ -0,0 +1,107 @@
+"""WeasyPrint engine.
+
+Default engine for documents we generate ourselves. It implements CSS Paged
+Media properly, which is what makes counters, running headers and repeated table
+headers work, and it uses far less memory than a headless browser.
+
+It does not execute JavaScript. That is intentional.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import logging
+from pathlib import Path
+
+from ..models import ConvertRequest
+from ..pdf.styles import build_page_css
+from .base import ChunkRender, RenderEngine
+
+logger = logging.getLogger(__name__)
+
+
+class WeasyPrintEngine(RenderEngine):
+    name = "weasyprint"
+
+    @property
+    def supports_anchor_pages(self) -> bool:
+        return True
+
+    async def available(self) -> bool:
+        try:
+            import weasyprint  # noqa: F401
+        except Exception as exc:  # noqa: BLE001 - reported, never silent
+            logger.error("WeasyPrint is not importable", exc_info=exc)
+            return False
+        return True
+
+    async def render_chunk(
+        self,
+        html: str,
+        base_url: str | None,
+        request: ConvertRequest,
+        output_path: Path,
+        total_pages: int | None = None,
+        asset_gate=None,
+    ) -> ChunkRender:
+        return await asyncio.to_thread(
+            self._render_blocking, html, base_url, request, output_path, total_pages, asset_gate
+        )
+
+    def _render_blocking(
+        self,
+        html: str,
+        base_url: str | None,
+        request: ConvertRequest,
+        output_path: Path,
+        total_pages: int | None,
+        asset_gate,
+    ) -> ChunkRender:
+        from weasyprint import CSS, HTML
+
+        kwargs = {"string": html, "base_url": base_url}
+        if asset_gate is not None:
+            kwargs["url_fetcher"] = asset_gate.weasy_fetcher()
+
+        stylesheets = []
+        if total_pages is not None:
+            # Only used when the page total is known up front, which happens on
+            # the second pass of an unchunked document.
+            stylesheets.append(
+                CSS(
+                    string=build_page_css(
+                        request.page,
+                        request.page_numbers,
+                        total_pages=total_pages,
+                        outline=request.outline,
+                    )
+                )
+            )
+
+        document = HTML(**kwargs).render(stylesheets=stylesheets or None)
+
+        write_kwargs = {}
+        if request.pdf_profile:
+            write_kwargs["pdf_variant"] = request.pdf_profile
+
+        document.write_pdf(target=str(output_path), **write_kwargs)
+
+        return ChunkRender(
+            path=output_path,
+            page_count=len(document.pages),
+            anchor_pages=self._anchor_pages(document),
+        )
+
+    @staticmethod
+    def _anchor_pages(document) -> dict[str, int]:
+        """Map anchor name to the zero based page index it landed on."""
+        anchors: dict[str, int] = {}
+        for index, page in enumerate(document.pages):
+            page_anchors = getattr(page, "anchors", None)
+            if not page_anchors:
+                continue
+            for name in page_anchors:
+                anchors.setdefault(name, index)
+        if not anchors:
+            logger.warning("WeasyPrint returned no anchors, table of contents page numbers may be missing")
+        return anchors