Extract db, models, ui, and provider-github into dedicated packages

[devarshishimpi]

Description

Continues the workspace restructuring started in #87, splitting more of src/server and src/client out of the monolithic app into standalone packages/apps:

• packages/db - all src/server/db/* modules (accounts, jobs, settings, repo-configs, migrations, repository classes) become a standalone package with its own package.json/tsconfig/vitest.config.
• packages/models - the model-provider layer (Anthropic, Google, OpenAI, Vertex, Cloudflare providers, chain runner, rate limits, catalog, schema handling) and its full spec suite move out of src/server/models.
• packages/provider-github - GitHub App auth, HTTP client, diff fetch, labels, OAuth, and review-post logic extracted into a dedicated provider package (continuing the pattern from the earlier GitHub-provider extraction).
• packages/ui - shared React components (button, input, select, layer-card, badges, chart primitives, motion helpers) and client-side libs (theme, highlight, markdown-plugins, file-tree) consolidated into a UI package consumed by src/client.
• apps/worker - the Cloudflare Worker entry point, wrangler config, env/session/orchestrator ports split out into its own app directory.
• Central constants.ts files added to packages/core, packages/db, and packages/schema to replace scattered default configs.

Part of #66
Closes #70 #71 #72 #73 #75

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore (refactoring, dependency updates, etc.)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

• Unit Tests
• Integration Tests
• Manual Dashboard Verification
• Manual GitHub Webhook Verification

Checklist:

• I have starred Codra on GitHub
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• I have signed the CLA

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