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/merger.py

@@ -0,0 +1,53 @@
+"""Merging of chunk PDFs and reading of basic page geometry."""
+
+from __future__ import annotations
+
+import logging
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+
+def merge(paths: list[Path], output_path: Path) -> int:
+    """Concatenate chunk PDFs into one file.
+
+    PdfWriter.append is used on purpose, it carries over bookmarks and internal
+    links and shifts their page references by the running offset.
+    """
+    from pypdf import PdfWriter
+
+    if not paths:
+        raise ValueError("Neni co slucovat, seznam PDF je prazdny.")
+
+    if len(paths) == 1:
+        paths[0].replace(output_path)
+        return page_count(output_path)
+
+    writer = PdfWriter()
+    try:
+        for path in paths:
+            writer.append(str(path))
+        with output_path.open("wb") as handle:
+            writer.write(handle)
+    finally:
+        writer.close()
+
+    total = page_count(output_path)
+    logger.info("Chunks merged", extra={"chunks": len(paths), "pages": total})
+    return total
+
+
+def page_count(path: Path) -> int:
+    from pypdf import PdfReader
+
+    with path.open("rb") as handle:
+        return len(PdfReader(handle).pages)
+
+
+def first_page_size(path: Path) -> tuple[float, float]:
+    """Width and height of the first page in points."""
+    from pypdf import PdfReader
+
+    with path.open("rb") as handle:
+        box = PdfReader(handle).pages[0].mediabox
+        return float(box.width), float(box.height)