fix: remove COPY output/ and fix PYTHONPATH in Dockerfile.chat

Resolve chat service build failure caused by missing output directory:

**Root Cause:**
- output/ directory is in .gitignore and not included in Docker build context
- COPY output/ /app/output/ failed with "not found" error
- PYTHONPATH still had undefined $PYTHONPATH variable

**Solution:**
- Remove COPY output/ line (directory created later with mkdir)
- Fix PYTHONPATH: /app/src:$PYTHONPATH → /app/src
- output/ directory already created at line 51: mkdir -p /app/output

**Errors Fixed:**
- ERROR: "/output": not found
- WARNING: Usage of undefined variable '$PYTHONPATH'

Successfully tested Docker build.

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

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

deploy/docker/Dockerfile.chat

@@ -36,7 +36,6 @@ RUN pip install --no-cache-dir -r requirements.txt
 COPY src/ /app/src/
 COPY config/ /app/config/
 COPY scripts/ /app/scripts/
-COPY output/ /app/output/
 COPY pyproject.toml README.md /app/
 
 # Install poetry-core (required for install with pyproject.toml)
@@ -46,7 +45,7 @@ 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/data /app/output /app/scripts