test: raise coverage above 60% in cmd/mediaplayer, model, and probe

- cmd/mediaplayer: add runWithSignal injection point for testing; cover
  version flag, invalid flags, invalid config, normal shutdown, all log
  levels, invalid DB, and privileged-port bind failure. Refactor run()
  to delegate to runWithSignal with optional signal channel.
- internal/config: allow PORT=0 (ephemeral) to support test server startup.
  Update AGENTS.md validation docs accordingly.
- internal/model: add comprehensive ScanProgress tests (Start, Done,
  IncrementFile/Set, SetCurrentSet/FilesTotal, Copy isolation, and
  concurrent access).
- internal/probe: add image tests for isImagePath (all extensions and
  case insensitivity), real EXIF extraction via ImageMagick + exiv2,
  Probe against real JPEG, MP4, empty file, and nonexistent paths.
- internal/config_test: replace PORT=0 invalid-value test with PORT=-1.

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