feat(backend/copilot): Copilot Executor Microservice (#12057)

Uncouple Copilot task execution from the REST API server. This should
help performance and scalability, and allows task execution to continue
regardless of the state of the user's connection.

- Resolves #12023

### Changes 🏗️

- Add `backend.copilot.executor`->`CoPilotExecutor` (setup similar to
`backend.executor`->`ExecutionManager`).

This executor service uses RabbitMQ-based task distribution, and sticks
with the existing Redis Streams setup for task output. It uses a cluster
lock mechanism to ensure a task is only executed by one pod, and the
`DatabaseManager` for pooled DB access.

- Add `backend.data.db_accessors` for automatic choice of direct/proxied
DB access

Chat requests now flow: API → RabbitMQ → CoPilot Executor → Redis
Streams → SSE Client. This enables horizontal scaling of chat processing
and isolates long-running LLM operations from the API service.

- Move non-API Copilot stuff into `backend.copilot` (from
`backend.api.features.chat`)
  - Updated import paths for all usages

- Move `backend.executor.database` to `backend.data.db_manager` and add
methods for copilot executor
  - Updated import paths for all usages
- Make `backend.copilot.db` RPC-compatible (-> DB ops return ~~Prisma~~
Pydantic models)
  - Make `backend.data.workspace` RPC-compatible
  - Make `backend.data.graphs.get_store_listed_graphs` RPC-compatible

DX:
- Add `copilot_executor` service to Docker setup

Config:
- Add `Config.num_copilot_workers` (default 5) and
`Config.copilot_executor_port` (default 8008)
- Remove unused `Config.agent_server_port`

> [!WARNING]
> **This change adds a new microservice to the system, with entrypoint
`backend.copilot.executor`.**
> The `docker compose` setup has been updated, but if you run the
Platform on something else, you'll have to update your deployment config
to include this new service.
>
> When running locally, the `CoPilotExecutor` uses port 8008 by default.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Copilot works
    - [x] Processes messages when triggered
    - [x] Can use its tools

#### For configuration changes:

- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

---------

Co-authored-by: Zamil Majdy <zamil.majdy@agpt.co>