feat: Move alembic config into mealie package for easier distribution

[chishm]

What type of PR is this?

• cleanup
• feature

What this PR does / why we need it:

The Alembic configuration and database migration scripts are moved inside the mealie Python package. This means they will be always available, in a consistent location. The advantages are:

• No extra step needed during mealie installation to add the Alembic configuration and scripts.
• No need to guess where the configuration is, so the ALEMBIC_CONFIG_FILE environment variable doesn't need to be set, ever. I've left it in as an option for people who still want to use it.

As a side effect, ruff (specifically isort) no longer thinks alembic is a project ("LOCALFOLDER") package, so the imports of alembic are moved to the third-party group.

Which issue(s) this PR fixes:

This is one task from discussion #4322.

Testing

I built and ran the Docker container, then made sure the database migration still occurred.

I created a database change following the instructions in database-changes.md.

[github-actions]

This PR is stale because it has been open 45 days with no activity.

[chishm]

I'm waiting on #4551 to be reviewed and merged before I rebase this PR for merge.

But if someone wants to review this one first, I'm happy to rebase now.

[hay-kot]

LGTM, please rebase and I'll get it merged!

[chishm]

Thank you for the review. I've rebased and re-run the manual tests (creating a database migration, building and running the Docker image) and I'm happy for this to be merged.

[tecosaur]

A quick question, after this change does this patch still look relevant? https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/me/mealie/package.nix#L94-L108

[chishm]

A quick question, after this change does this patch still look relevant? https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/me/mealie/package.nix#L94-L108

Taking a look, I think lines 95-98 are still relevant, but 100-107 shouldn't be necessary any more.