feat(embeddings): core embedding generation services and tasks

[EmanueleDeRossi1]

Purpose

This PR introduces the core services and Celery tasks for generating and managing embeddings for any embeddable entity in the system.

What Changed

• EmbeddingGenerator: A generic generator that handles the deduplication logic (via configuration and text hashing), interacts with the SDK for vector generation, and manages the lifecycle of embeddings (marking older ones as stale).
• EmbeddingService: An orchestration service that determines whether an embedding should be generated synchronously or asynchronously (via Celery). It also handles model resolution from user settings.
• generate_embedding Celery Task: A background task that executes the embedding generation in a worker, ensuring proper tenant context isolation.