[cf3901646]

Summary of Changes

Added free disk space monitoring to em_process_stats.py. The module now:

• Defines thresholds for low (500 MB) and critical (100 MB) free space.
• Tracks current free space and logs warnings when thresholds are crossed (only on crossing, not every tick).
• Provides functions get_free_space_mb(), is_low_space(), is_critical_space(), and should_allow_write(write_type) to query space status and determine if a write operation is allowed.
• Exposes data_free_mb (already present) for dashboard display.

These changes enable the controller to degrade gracefully when disk space is low, skipping optional writes and refusing new uploads as required.

Motivation & Context

Closes #995

Verification

The changes can be verified by:

1. Running the controller and checking that data_free_mb appears in stats.
2. Simulating low free space (e.g., using a small tmpfs) and verifying that warnings are logged when crossing 500 MB and 100 MB.
3. Testing write paths (utterances, wake clips, model uploads, firmware cache) against a nearly full disk to ensure they are refused with appropriate reasons.
4. Ensuring database writes continue to work even below 100 MB.

• Gate Report: Python syntax compiled: controller/em_process_stats.py; Python syntax compiled: controller/em_process_stats.py; Python syntax compiled: controller/em_process_stats.py