Support Immich URL/API key via docker-compose env vars

docker-compose.yml is tracked in a repo meant for publishing, so it can't
hold a real API key. Renamed it to docker-compose.yml.example (placeholder
values, safe to commit) and gitignored the real docker-compose.yml --
deploying is now "cp the example, fill in real values, docker compose up",
no .env file needed.

config.load() now reads IMMICH_URL/IMMICH_API_KEY from the environment
and applies them on top of whatever's in config.json, so setting them in
the compose file's environment: block takes effect without ever touching
the web UI. Env vars always win over the UI-saved values when both are
present -- verified they survive a save() with different UI-entered
values still in place.

.gitignore

@@ -11,6 +11,9 @@ server/**/__pycache__/
 server/.venv/
 server/*.egg-info/
 server/data/
+# Real deploy config, copied from docker-compose.yml.example -- holds the
+# Immich API key, must never be committed.
+server/docker-compose.yml
 .env
 
 # Editor / OS cruft