feat: DB Migrations

[andypols]

Description

This PR provides adds DB migrations. #1656 adds dateCreated and lastModified to repository records to support sorting or repository by date. This cannot be used without a corresponding migration of existing data.

Each migration is tracked by its own unique ID, rather than relying on a single global “current database version”. This means two branches can add separate migrations and be merged in either order. When both reach main, GitProxy checks the complete registry and runs whichever migration IDs have not yet been applied.

This gives us:

• a clear history of data changes
• one shared implementation across database backends
• a record of which changes have already run
• safe restart behaviour after a partial migration
• optional rollback support
• a consistent pattern for future schema and data changes

Example migration

For example a migration for #1656 could be implemented like this.

import type { Repo } from '../types';
import type { Migration } from './index';

export const addRepoDates: Migration = {
  id: '20260722-add-repo-dates',
  up: async (sink) => { 
    const repos = await sink.getRepos(); 
    for (const repo of repos) {
      if (repo.dateCreated && repo.lastModified) { 
        continue; 
      }

      const dateCreated =
        repo.dateCreated ?? (await sink.deriveCreatedAt(repo._id)) ?? new Date().toISOString();

      await sink.updateRepo({
          ...repo, 
          dateCreated, 
          lastModified: repo.lastModified ?? dateCreated, 
      });
    } 
  }, 
};

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