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/styles.py

@@ -0,0 +1,115 @@
+"""Generation of the page stylesheet injected into the source document."""
+
+from __future__ import annotations
+
+import re
+
+from ..models import PageNumbers, PageSettings
+
+NAMED_SIZE = re.compile(r"^[A-Za-z][A-Za-z0-9]*$")
+
+MARGIN_BOXES = {
+    "top-left": "@top-left",
+    "top-center": "@top-center",
+    "top-right": "@top-right",
+    "bottom-left": "@bottom-left",
+    "bottom-center": "@bottom-center",
+    "bottom-right": "@bottom-right",
+}
+
+PLACEHOLDER = re.compile(r"(\{page\}|\{pages\})")
+
+
+def page_size_value(page: PageSettings) -> str:
+    fmt = page.format.strip()
+    if NAMED_SIZE.match(fmt):
+        return f"{fmt} {page.orientation}"
+    # Explicit dimensions already carry the orientation.
+    return fmt
+
+
+def margin_shorthand(page: PageSettings) -> str:
+    m = page.margin
+    return f"{m.top} {m.right} {m.bottom} {m.left}"
+
+
+def css_content_value(fmt: str, total_pages: int | None) -> str:
+    """Turn "{page} / {pages}" into a CSS content value.
+
+    When total_pages is known the total is written as a literal, otherwise the
+    CSS counter(pages) is used.
+    """
+    parts: list[str] = []
+    for token in PLACEHOLDER.split(fmt):
+        if token == "{page}":
+            parts.append("counter(page)")
+        elif token == "{pages}":
+            parts.append(str(total_pages) if total_pages is not None else "counter(pages)")
+        elif token:
+            escaped = token.replace("\\", "\\\\").replace('"', '\\"')
+            parts.append(f'"{escaped}"')
+    return " ".join(parts) if parts else '""'
+
+
+def build_page_css(
+    page: PageSettings,
+    page_numbers: PageNumbers | None = None,
+    total_pages: int | None = None,
+    outline: bool = True,
+) -> str:
+    """Stylesheet applied on top of the document styles."""
+
+    rules = [
+        "@page {",
+        f"  size: {page_size_value(page)};",
+        f"  margin: {margin_shorthand(page)};",
+    ]
+
+    if page_numbers is not None and page_numbers.enabled:
+        box = MARGIN_BOXES[page_numbers.position]
+        rules.append(f"  {box} {{")
+        rules.append(f"    content: {css_content_value(page_numbers.format, total_pages)};")
+        rules.append("    font-size: 9pt;")
+        rules.append("    color: #444;")
+        rules.append("  }")
+
+    rules.append("}")
+
+    if not outline:
+        rules.append("h1, h2, h3, h4, h5, h6 { bookmark-level: none; }")
+
+    return "\n".join(rules)
+
+
+def build_overlay_css(
+    width_pt: float,
+    height_pt: float,
+    page: PageSettings,
+    page_numbers: PageNumbers,
+    total_pages: int,
+) -> str:
+    """Stylesheet for the transparent numbering layer merged onto the final PDF.
+
+    The size comes from the produced PDF itself, so the overlay always matches
+    even when the source document declares its own @page size.
+    """
+    box = MARGIN_BOXES[page_numbers.position]
+    reset = ""
+    if page_numbers.start_at != 1:
+        reset = f"body {{ counter-reset: page {page_numbers.start_at - 1}; }}\n"
+
+    return (
+        f"@page {{\n"
+        f"  size: {width_pt:.2f}pt {height_pt:.2f}pt;\n"
+        f"  margin: {margin_shorthand(page)};\n"
+        f"  {box} {{\n"
+        f"    content: {css_content_value(page_numbers.format, total_pages)};\n"
+        f"    font-size: 9pt;\n"
+        f"    color: #444;\n"
+        f"  }}\n"
+        f"}}\n"
+        f"{reset}"
+        f"body {{ margin: 0; }}\n"
+        f".pdf-page-slot {{ height: 1px; break-after: page; }}\n"
+        f".pdf-page-slot:last-child {{ break-after: auto; }}\n"
+    )