Make xhtml2pdf optional, fix install on Linux without libcairo

Move xhtml2pdf to the new [pdf] extra so default `pip install maigret`
no longer pulls pycairo (which has no Linux/macOS wheels and breaks the
build without libcairo2-dev). save_pdf_report now raises a clear
RuntimeError pointing to `pip install 'maigret[pdf]'`, and the CLI
turns it into a friendly warning instead of a crash. Adds tests
covering the missing-extra path, plus per-OS install docs.

Fix for #2657, #2534

pyproject.toml

@@ -69,7 +69,7 @@ torrequest = "^0.1.0"
 alive_progress = "^3.2.0"
 typing-extensions = "^4.14.1"
 webencodings = "^0.5.1"
-xhtml2pdf = "^0.2.11"
+xhtml2pdf = {version = "^0.2.11", optional = true}
 XMind = "^1.2.0"
 yarl = "^1.20.1"
 networkx = "^2.6.3"
@@ -82,6 +82,13 @@ platformdirs = "^4.3.8"
 curl-cffi = ">=0.14,<1.0"
 
 
+[tool.poetry.extras]
+# Install PDF support with: pip install 'maigret[pdf]'
+# Skipped by default because the underlying `pycairo` has no Linux/macOS
+# wheels on PyPI and requires system libcairo + pkg-config to build.
+pdf = ["xhtml2pdf"]
+
+
 [tool.poetry.group.dev.dependencies]
 # How to add a new dev dependency: poetry add black --group dev
 # Install dev dependencies with: poetry install --with dev
@@ -92,6 +99,7 @@ pytest-cov = ">=6,<8"
 pytest-httpserver = "^1.0.0"
 pytest-rerunfailures = ">=15.1,<17.0"
 reportlab = "^4.4.3"
+xhtml2pdf = "^0.2.11"
 mypy = ">=1.14.1,<3.0.0"
 tuna = "^0.5.11"
 coverage = "^7.9.2"