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/pdf/paginator.py

@@ -0,0 +1,80 @@
+"""Page numbering of the merged document.
+
+Two ways to number pages:
+
+css
+    CSS counters do the work during the render. Correct and cheap, but it only
+    works when the whole document is rendered in one pass, because each chunk
+    restarts the page counter.
+
+overlay
+    A transparent numbering layer with the same page size is rendered once and
+    merged onto the finished PDF. This is the only option for a chunked document
+    and for Chromium, which has no usable page counters.
+"""
+
+from __future__ import annotations
+
+import logging
+from pathlib import Path
+
+from ..errors import EngineUnavailableError
+from ..models import PageNumbers, PageSettings
+from .styles import build_overlay_css
+
+logger = logging.getLogger(__name__)
+
+
+def build_overlay(
+    total_pages: int,
+    width_pt: float,
+    height_pt: float,
+    page: PageSettings,
+    page_numbers: PageNumbers,
+    output_path: Path,
+) -> Path:
+    """Render the numbering layer, one empty page per page of the document."""
+    try:
+        from weasyprint import HTML
+    except ImportError as exc:
+        raise EngineUnavailableError(
+            "Cislovani stranek vyzaduje nainstalovany WeasyPrint, ktery kresli cislovaci vrstvu.",
+        ) from exc
+
+    css = build_overlay_css(width_pt, height_pt, page, page_numbers, total_pages)
+    slots = '<div class="pdf-page-slot"></div>' * total_pages
+    html = (
+        "<!DOCTYPE html><html><head><meta charset=\"utf-8\">"
+        f"<style>{css}</style></head><body>{slots}</body></html>"
+    )
+
+    HTML(string=html).write_pdf(target=str(output_path))
+    logger.info("Numbering overlay rendered", extra={"pages": total_pages})
+    return output_path
+
+
+def apply_overlay(document_path: Path, overlay_path: Path, output_path: Path) -> None:
+    """Stamp the numbering layer onto every page of the document."""
+    from pypdf import PdfReader, PdfWriter
+
+    writer = PdfWriter(clone_from=str(document_path))
+    try:
+        with overlay_path.open("rb") as handle:
+            overlay = PdfReader(handle)
+            available = len(overlay.pages)
+
+            if available < len(writer.pages):
+                logger.warning(
+                    "Numbering overlay has fewer pages than the document, tail will stay unnumbered",
+                    extra={"overlay_pages": available, "document_pages": len(writer.pages)},
+                )
+
+            for index, page in enumerate(writer.pages):
+                if index >= available:
+                    break
+                page.merge_page(overlay.pages[index])
+
+            with output_path.open("wb") as target:
+                writer.write(target)
+    finally:
+        writer.close()