Feature: Implement report.screenshot() support

[KCarretto]

Implemented full-stack screenshot support. The agent now has a report.screenshot() method that captures all monitors and streams them to the C2 server. The backend stores these in a dedicated Screenshot entity and provides an authenticated download endpoint.

Testing:

• Verified backend ReportFile logic with unit tests (tavern/internal/c2/api_report_file_test.go).
• Verified Rust agent compilation with cargo check.
• Verified backend compilation with go build.
• Infrastructure updates ensure CI/CD compatibility.

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PR created automatically by Jules for task 12489053985665727235 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2686    ±0	2686    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1302	✅	2686	2686	0	0	0	0	44.9s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2686	0	0	5.6s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
TestDockerExecutor_Build_ContextCancellation	1	5.6s	5.6s
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.2s	5.2s
TestInteractiveShell	1	5.1s	5.1s
TestOtherStreamOutput	1	5.0s	5.0s
imix::bin/imix: install::tests::test_install_execution	3	1.5s	4.5s
imix::bin/imix: install::tests::test_install_execution	3	1.5s	4.5s
imix::bin/imix: install::tests::test_install_execution	3	1.5s	4.5s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚

🔄 This comment has been updated