feat: add on-disk media integrity checks and auto-remediation

[naveedious]

Summary

Extends Episode Guard to check existing on-disk episodes during lookahead cycles for corruption, truncation, or bad releases, and optionally auto-remediate via Sonarr.

Details

• Tier 1 (Runtime Validation): Compares physical file duration against expected duration from Sonarr metadata (flagged if < 80% tolerance threshold).
• Tier 2 (Stream & Tail Probe): Uses ffprobe and ffmpeg via bounded subprocesses to decode the final 30 seconds of the stream to /dev/null (-f null -), catching broken containers and EOF truncation without memory bloat.
• Path Mapping & Graceful Fallback: Dynamic path prefix translation from Sonarr (e.g. /data/TV) to local container mounts (/tv), falling back to Sonarr API metadata if unmounted.
• SQLite Verification Cache: Added verified_files table to prevent repeated I/O on already-verified files.
• Remediation & Notifications: Added configurable setting (auto_remediate [deletes file via Sonarr API + triggers replacement search] vs notify_only vs disabled) plus Apprise alerts.
• UI & Container: Added settings controls in Settings UI, event badges in Activity Log, installed ffmpeg in runtime image, and updated documentation.