Scaffold rgl-photo-backup with output helpers and test harness

Start the script that will back up the Randomness Guide to London's photos
to disk and Immich. This commit holds the constants, error classes and
quiet-aware output helpers every later section relies on, plus the test
loader for an extensionless script.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1 @@
+__pycache__/