fix: add poetry-plugin-export and fix PYTHONPATH in Dockerfiles

- Install poetry-plugin-export alongside Poetry 1.8.0 in all Python service Dockerfiles (api, chat, worker)
- Fix PYTHONPATH environment variable to avoid undefined variable warnings
- Resolves Docker build pipeline failure where poetry export command was not available

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

deploy/docker/Dockerfile.api

@@ -3,8 +3,8 @@ FROM python:3.12-slim AS builder
 
 WORKDIR /build
 
-# Install Poetry
-RUN pip install --no-cache-dir poetry==1.8.0
+# Install Poetry and export plugin
+RUN pip install --no-cache-dir poetry==1.8.0 poetry-plugin-export
 
 # Copy dependency files
 COPY pyproject.toml poetry.lock ./
@@ -48,7 +48,7 @@ RUN pip install --no-cache-dir poetry-core
 RUN pip install --no-cache-dir /app
 
 # Set PYTHONPATH to ensure module can be imported
-ENV PYTHONPATH=/app/src:$PYTHONPATH
+ENV PYTHONPATH=/app/src
 
 # Create necessary directories
 RUN mkdir -p /app/logs /app/output