Add external render progress API and fix C++ build paths

[azuer88]

Summary

• Add /renderProgress API endpoint — allows external before/after render scripts to push progress updates to the Octolapse progress bar in the OctoPrint UI. Useful for scripts
  that do their own processing (re-encoding, watermarking, masking) and want to drive the UI progress independently. Requires an Application Key to post.
• Fix setup.py C++ source paths — sources were referencing octoprint_octolapse/*.cpp but files live in octoprint_octolapse/data/lib/c/. Also fixes a bug where . in
  requirements.txt was being passed to install_requires.
• Add examples/ — usage example for the new API endpoint.

Changes

octoprint_octolapse/__init__.py

• Add POST /renderProgress Blueprint route accepting {"percent": float}, forwarded to the UI via the existing render progress message system.

octoprint_octolapse/render.py

• Add get_current_rendering_job() to RenderingProcessor so the progress endpoint can include job info in updates.

setup.py

• Fix C++ extension sources to use correct octoprint_octolapse/data/lib/c/ paths.
• Skip . entry when reading requirements.txt into install_requires.

examples/

• New example scripts demonstrating how to POST progress updates from an external shell or Python script.

Test plan

• POST /plugin/octolapse/renderProgress with {"percent": 50} returns {"success": true}
• Progress bar updates in OctoPrint UI when endpoint is called during a render
• pip install -e . succeeds without errors about missing C++ sources