feat: add Google Gemini Embedding support with hybrid provider system

[clls1-stinger]

This PR introduces support for Google Gemini Embeddings as an alternative to the local sentence-transformers engine.

Key Features:

• Hybrid Provider System: Seamlessly switch between local and cloud-based embeddings.
• Google Gemini Integration: Use google-generativeai for high-quality, high-dimension (768) embeddings.
• Smart Batching: Implemented automatic batching (100 items per request) to respect API quotas.
• Graceful Fallback: Automatically detects available providers (Local vs Google) based on environment variables (GOOGLE_API_KEY).
• SQLite Schema Migration: Added a provider column to the embeddings table to allow multi-model coexistence in the same project graph.

This enhancement allows users to scale their semantic search capabilities while keeping the default local-first privacy when desired.