feat: add @endpoint decorator for custom ModelAdmin FastAPI routes

[borhanst]

Closes #49

Summary

Implements issue #49: add an @endpoint decorator for ModelAdmin classes that registers arbitrary FastAPI routes on a model's admin router, with full add_api_route() configuration support.

Changes

• fastapi_admin_kit/admin/decorators.py — new EndpointOptions dataclass + endpoint() decorator. Stores config on the method as _admin_endpoint (mirrors the existing @column / @action pattern).
• fastapi_admin_kit/router.py — build_model_router() auto-discovers _admin_endpoint methods and registers them via router.add_api_route(). Routes are registered before the /{id} catch-all so paths like /health-check are never swallowed by the edit route. An untyped request parameter is injected as the FastAPI Request (matching the documented usage async def health_check(self, request)).
• RBAC — permission="view" adds a require_permission(<model>, <permission>) dependency; arbitrary dependencies=[Depends(...)] are also supported.
• Exports — endpoint exported from fastapi_admin_kit and exposed as ModelAdmin.endpoint.
• Pathing — endpoints are served under /<model_name>/<path> (e.g. /admin/products/health-check).
• Tests — 16 tests covering metadata, registration, ordering vs. /{id}, untyped/typed request, query params, POST methods, RBAC denial, and OpenAPI docs.
• Docs — docs/guide/model-registration.md and docs/guide/features.md updated.

Example

from fastapi_admin_kit import endpoint

@admin.register(Product)
class ProductAdmin(ModelAdmin):
    @endpoint(
        path="/health-check",
        methods=["GET"],
        tags=["monitoring"],
        description="Health check endpoint",
        permission="view",
    )
    async def health_check(self, request):
        return {"status": "healthy"}

Verification

• ruff check passes
• pytest — 846 passed, 3 skipped (full suite)