feat(server): the instance names where its source is

Both licences (AGPL for the app, SSPL for the server) require offering the source of what an instance runs to the people who use it. instance.info() now carries sourceCodeUrl: the operator's SOURCE_URL, which they must set when running modified code, and otherwise this project's repository. Serverpod validates its configuration files strictly, so the value comes from the environment; deploy/compose.yaml passes it on from deploy/.env, whose example explains it. A test covers the default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/compose.yaml

@@ -50,6 +50,8 @@ services:
       # GitHub sign-in, only with this instance's GitHub OAuth app; empty: off.
       SERVERPOD_PASSWORD_githubClientId: ${GITHUB_CLIENT_ID:-}
       SERVERPOD_PASSWORD_githubClientSecret: ${GITHUB_CLIENT_SECRET:-}
+      # Where the source of what this instance runs is; empty: the project's.
+      SOURCE_URL: ${SOURCE_URL:-}
     depends_on:
       postgres:
         condition: service_healthy