Rationalize optional dependencies vs dependency groups

Establish clear separation between user-facing optional dependencies
and developer-only dependency groups:

**Optional Dependencies (user features):**
- watcher: File watching service for batch processing
- webservice: Streamlit-based web UI
- Installable via: uv sync --extra <name> or pip install ocrmypdf[name]

**Dependency Groups (developer tools):**
- test: Testing infrastructure (merged from test + extended_test)
- docs: Documentation building tools
- streamlit-dev: Enhanced Streamlit development tools
- dev: General development tools (mypy, ipykernel)
- Installable via: uv sync --group <name> (uv only, NOT pip)

Breaking changes for developers:
- pip install -e .[test] no longer works → use uv sync --group test
- pip install -e .[docs] no longer works → use uv sync --group docs
- pip install -e .[extended_test] removed → merged into test group

No breaking changes for end users:
- pip install ocrmypdf[watcher] still works
- pip install ocrmypdf[webservice] still works

Updated:
- CI/CD workflows to use uv sync --group test
- Docker images to exclude test dependencies
- Documentation to recommend uv with pip as fallback
- pyproject.toml with clear comments explaining both systems

.github/workflows/build.yml

@@ -74,7 +74,7 @@ jobs:
 
       - name: Install Python packages
         run: |
-          uv sync --extra test --no-dev
+          uv sync --group test
 
       - name: Report versions
         run: |
@@ -137,7 +137,7 @@ jobs:
 
       - name: Install Python packages
         run: |
-          uv sync --extra test --no-dev
+          uv sync --group test
 
       - name: Report versions
         run: |
@@ -192,7 +192,7 @@ jobs:
 
       - name: Install Python packages
         run: |
-          uv sync --extra test --no-dev
+          uv sync --group test
 
       - name: Test
         run: |