⚙️ FEATURE-#239: Implement Server provider for remote API communication

[FernandoCelmer]

Description

• dotflow/abc/server.py — New Server ABC with create_workflow, update_workflow, create_task, update_task
• dotflow/providers/server_default.py — ServerDefault HTTP client for dotflow-api
• dotflow/core/config.py — Integrated server param, removed api
• dotflow/core/dotflow.py — Hook server.create_workflow on init
• dotflow/core/task.py — Hook server.create_task on init, server.update_task on status change (non-blocking for intermediate, synchronous for final)
• dotflow/core/engine.py — Set started_at/finished_at timestamps
• dotflow/core/workflow.py — Hook server.update_workflow on start (In progress) and finish (Completed/Failed)
• dotflow/core/types/workflow.py — Added FAILED status to WorkflowStatus
• dotflow/core/serializers/task.py — Refactored: remove double-parse, add computed_field error, add datetime fields, fix type hints
• dotflow/core/serializers/workflow.py — Simplified (removed custom model_dump_json)
• dotflow/core/serializers/transport.py — Fixed typo resourcec → set_resource
• dotflow/providers/__init__.py — Export ServerDefault
• dotflow/abc/api.py — Removed (deprecated)
• dotflow/providers/api_default.py — Removed (deprecated)
• tests/providers/test_server_default.py — Tests for ServerDefault
• tests/core/test_config.py — Updated for server provider
• tests/core/test_task.py — Updated for datetime fields
• tests/core/test_task_build.py — Updated for datetime fields
• docs_src/server/ — 3 usage examples
• README.md — Server provider documentation

Motivation and Context

The dotflow lib needed a proper provider to send real-time execution data (workflow status, task status, errors, duration, context) to a remote server like dotflow-api. The existing Api/ApiDefault was incomplete (most methods were no-ops).

Closes #239

Types of changes

• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly