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/errors.py

@@ -0,0 +1,103 @@
+"""Application errors.
+
+Every failure surfaces a machine readable error_code and a message in Czech,
+because the message is shown to the caller.
+"""
+
+from __future__ import annotations
+
+
+class ConversionError(Exception):
+    """Base class for every error the conversion pipeline can raise."""
+
+    error_code = "internal_error"
+    status_code = 500
+
+    def __init__(self, message: str, detail: dict | None = None) -> None:
+        super().__init__(message)
+        self.message = message
+        self.detail = detail or {}
+
+    def to_dict(self) -> dict:
+        payload = {"error_code": self.error_code, "message": self.message}
+        if self.detail:
+            payload["detail"] = self.detail
+        return payload
+
+
+class InvalidRequestError(ConversionError):
+    error_code = "invalid_request"
+    status_code = 400
+
+
+class BlockedTargetError(ConversionError):
+    """The requested URL points somewhere the service refuses to reach."""
+
+    error_code = "blocked_target"
+    status_code = 400
+
+
+class SourceUnavailableError(ConversionError):
+    error_code = "source_unavailable"
+    status_code = 502
+
+
+class RenderTimeoutError(ConversionError):
+    error_code = "render_timeout"
+    status_code = 504
+
+
+class SyncTooLongError(ConversionError):
+    """Synchronous conversion exceeded its budget, caller should use /jobs."""
+
+    error_code = "sync_too_long"
+    status_code = 413
+
+
+class EngineUnavailableError(ConversionError):
+    error_code = "engine_unavailable"
+    status_code = 503
+
+
+class UnsupportedCombinationError(ConversionError):
+    error_code = "unsupported_combination"
+    status_code = 400
+
+
+class LimitExceededError(ConversionError):
+    error_code = "limit_exceeded"
+    status_code = 400
+
+
+class JobNotFoundError(ConversionError):
+    error_code = "job_not_found"
+    status_code = 404
+
+
+class QueueFullError(ConversionError):
+    error_code = "queue_full"
+    status_code = 503
+
+
+ERROR_CLASSES = (
+    InvalidRequestError,
+    BlockedTargetError,
+    SourceUnavailableError,
+    RenderTimeoutError,
+    SyncTooLongError,
+    EngineUnavailableError,
+    UnsupportedCombinationError,
+    LimitExceededError,
+    JobNotFoundError,
+    QueueFullError,
+)
+
+STATUS_BY_CODE = {cls.error_code: cls.status_code for cls in ERROR_CLASSES}
+
+
+def error_from_code(error_code: str, message: str, detail: dict | None = None) -> ConversionError:
+    """Rebuild a typed error from a stored job error."""
+    error = ConversionError(message, detail)
+    error.error_code = error_code
+    error.status_code = STATUS_BY_CODE.get(error_code, 500)
+    return error