Fix(ci) integration tests

[nebay-abraha]

• The workflow triggered on master, but the default branch is main, so it never ran on pushes or PRs. Both triggers now point to main.
• The rspace-web build needs src/main/resources/deployments/dev/deployment.properties. The workflow now copies it from the checked-in .example file before building.
• We used to built rspace-web backend only without the frontend, now both frontend and backend are build.

MediaFileContentValidator

• 5 of the 96 integration tests that ran failed all test_import_tree*. Their fixture rspace_client/tests/data/tree/subdir/b.png was an empty 0-byte file. Newer rspace-web runs uploads through MediaFileContentValidator, which rejects a file when its content doesn't match an image extension. Empty bytes don't look like a PNG, so the upload got a 422.

[mKowalski256]

Looks good to me!