docs: clean up astrolabe references after extraction

Remove astrolabe-specific docs and sections that belong in the
astrolabe repo. Update remaining references to point to the
astrolabe repo where appropriate.

- Fix .gitmodules SSH → HTTPS URL for astrolabe submodule
- Remove bump-version.yml stale "astrolabe" scope comment
- Delete blog-introducing-astrolabe.md (moved to astrolabe repo)
- Remove "Astrolabe Background Token Refresh" section from auth-flows.md
- Replace "Astrolabe User Setup" section in authentication.md with link
- Remove "Astrolabe Internal URL" section from configuration.md
- Remove "Webhook Presets (via Astrolabe UI)" from webhook guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/configuration.md

@@ -531,28 +531,6 @@ docker-compose up
 
 ---
 
-## Astrolabe Internal URL
-
-The Astrolabe Nextcloud app may need to make internal HTTP requests to the local web server (e.g., for OAuth token refresh). By default, it uses `http://localhost` which works for standard Docker containers where PHP and Apache run together.
-
-| Variable | Description | Default |
-|----------|-------------|---------|
-| `astrolabe_internal_url` | Internal URL for server-to-server requests within container | `http://localhost` |
-
-**When to configure:**
-- Custom container setups where the internal web server is not on `localhost:80`
-- Kubernetes deployments with service discovery
-- Multi-container setups with separate web server containers
-
-**Example (Nextcloud config.php):**
-```php
-'astrolabe_internal_url' => 'http://web-server.internal:8080',
-```
-
-**Note:** This is for internal PHP-to-Apache requests, NOT for external client URLs. The default (`http://localhost`) works for standard Docker containers where PHP and Apache run together.
-
----
-
 ## Loading Environment Variables
 
 After creating your `.env` file, load the environment variables: