Set up PostgreSQL with pgvector extension

[VolenNakov]

Migrate from SQLite to PostgreSQL with pgvector support for AI-powered features.

Acceptance Criteria:

• PostgreSQL 15+ installed with pgvector extension
• Docker configuration updated for PostgreSQL
• Environment variables configured
• Database connection tested

Dependencies:

• Update go.mod with PostgreSQL dependencies
• Configure local development environment

Files to Create/Update:

• go.mod (add PostgreSQL dependencies)
• cmd/api/main.go (update database connection)
• internal/config/ (PostgreSQL configuration)
• .env (PostgreSQL connection string)