Refresh training reports before post-upload scripts

[bghira]

The static report embeds a snapshot refreshed every 100 metric records. Post-upload scripts could therefore copy an HTML report from step 943 even after metrics and validation images for checkpoint 1000 had been written.

Refresh the enabled local report before submitting an external hook. This reuses the shared path used by trainer uploads, validation publishing, and checkpoint hooks, so the report includes the latest recorded metrics, validation media, and timestep samples. Serialize report renders to prevent periodic and background upload refreshes from sharing a temporary file or overwriting a newer snapshot with an older one. Runs without the local tracker do not create reports.

Validation:

• The new hook regression failed before the fix with report step 943 instead of 1002, then passed with checkpoint-1000 validation media included. The hook's current-step placeholder remains 1002.
• The concurrent-render regression failed before the fix with an older snapshot replacing the latest, then passed after serializing renders.
• .venv/bin/python -m unittest -v -f tests.test_script_runner tests.test_local_metrics passed all 14 tests.
• .venv/bin/python -m unittest -v -f tests.test_trainer tests.test_validation_external tests.test_training_metrics_service tests.test_training_metrics_routes passed all 132 tests.
• git diff --cached --check passed.

Closes #3207.