refactor: restructure into true monorepo with turborepo and workspace packages

[mikais13]

Description

This PR restructures the repository into a true monorepo by introducing Bun workspaces, Turborepo, and splitting the codebase into distinct apps/api and packages/core workspace packages.

• converts the root package.json to a private workspace root with workspaces: ["apps/*", "packages/*"]
• moves the API source, Dockerfile, and fly.toml into apps/api/ with its own package.json and tsconfig.json
• extracts shared business logic (application services, GitHub client, models, schemas) into a new packages/core package published as @pr-stack/core
• adds a turbo.json with task definitions for build, dev, types:check, lint:check, and lint:fix, replacing the manual per-app scripts in the root
• updates root scripts to delegate to turbo run with optional --filter=api for targeted execution
• adds a bun build script in packages/core for the bundler output
• removes upstream dependsOn from turbo tasks that don't require it to keep task scheduling lean
• updates the Dockerfile to install dependencies from the workspace root before building the api app
• updates .github/workflows/fly-deploy.yml to reflect the moved Dockerfile path
• adds @pr-stack/core path alias in the root tsconfig.json pointing to packages/core/src for type checking

Not related to a ticket

How to test this change

Run the dev server via the root workspace:

bun dev

or for the API only:

bun dev:api

Verify the build works end-to-end:

bun build

Check that turbo correctly resolves the @pr-stack/core workspace dependency in apps/api and that types pass with bun types:check.