fix(pdf): replace fpdf2 with weasyprint for proper md→PDF rendering

fpdf2's built-in Helvetica was Latin-1 only and required manual line-by-line
markdown parsing, causing indentation artifacts at speaker changes. weasyprint
renders via HTML+CSS so layout, wrapping, and Unicode all work correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -3,6 +3,13 @@ FROM docker.io/library/python:3.12-slim
 ENV PYTHONDONTWRITEBYTECODE=1 \
     PYTHONUNBUFFERED=1
 
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libcairo2 \
+    libpango-1.0-0 \
+    libpangocairo-1.0-0 \
+    fonts-dejavu-core \
+    && rm -rf /var/lib/apt/lists/*
+
 WORKDIR /app
 COPY requirements.txt /app/requirements.txt
 RUN pip install --no-cache-dir -r /app/requirements.txt