[feature] Use maintenance router to serve 503 while server is starting/migrating

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

This pull request adds a simple barebones router that's initialized during startup, hangs around until db migrations are complete, and then is closed and replaced with the "main" server router.

The maintenance router serves static assets, serves 200 OK for "live" checks (not "ready" checks), and serves a 503 page for everything else, with Retry-After set to 2 minutes.

Should make things a little clearer for users when an instance is down running database migrations :) And should reassure admins that stuff is working, as well.

The command gotosocial server maintenance has also been added just to test the maintenance mode.

Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

damn, nice stuff 👀