feat(core): Add PostgreSQL (with pgvector) as vector engine backend

[YuweiXiao]

With this PR, users can now utilize claude-context in a fully local manner, powered by a local Ollama embedding service and vector engine provided by a local PostgreSQL docker container. All claude context-related operations occur entirely locally, with zero code transmission over the internet.

To enable PostgreSQL as vector backend, set through

VECTOR_DATABASE_PROVIDER = "postgres"
POSTGRES_CONNECTION_STRING = "postgresql://username:password@host:port/db"

[tan-yong-sheng]

+1 hope for this support, thanks a lot for the work.