FEAT Persist OpenAI target configurations and restore them from the database

[behnam-o]

Adds database-backed persistence for OpenAI target configurations.

• Introduces OpenAITargetConfig and its memory entry for storing registry name, endpoint, model, authentication mode, and API key reference.
• Updates backend target creation to persist supported targets in addition to registering them.
• Stores API keys in a separately configured Azure Key Vault and persists only their references.
• Adds a dbtargets initializer to reconstruct persisted targets and register them at startup.
• Keeps persistence behind the memory interface rather than exposing SQLAlchemy models to backend services.