Migrate to genai backend with API key configuration

Major improvements:
- Replaced custom HTTP clients with genai crate (80% code reduction)
- Support for 10+ AI providers automatically (OpenAI, Gemini, Anthropic, etc)
- Added flexible API key configuration through SQL:
  - Simple format: 'provider:key'
  - JSON format with explicit keys
  - rembed_client_options function
  - Environment variables (backward compatible)
- Added async runtime management with tokio
- Maintained full backward compatibility
- Prepared for batch processing support via embed_batch()

This migration dramatically simplifies the codebase while adding
more provider support and features.

Author: Ryan Malloy

API_KEY_GUIDE.md

@@ -0,0 +1,196 @@
+# API Key Configuration Guide
+
+With the new genai backend, sqlite-rembed offers multiple flexible ways to configure API keys directly through SQL, eliminating the need to set environment variables.
+
+## 🔑 API Key Configuration Methods
+
+### Method 1: Simple Provider:Key Format
+The easiest way - just use `provider:your-api-key`:
+
+```sql
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-openai', 'openai:sk-proj-abc123...'),
+  ('my-gemini', 'gemini:AIza...'),
+  ('my-groq', 'groq:gsk_abc123...');
+```
+
+### Method 2: JSON Configuration
+More explicit with JSON format:
+
+```sql
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-client', '{"provider": "openai", "api_key": "sk-proj-abc123..."}');
+
+-- Or specify the full model
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-client', '{"model": "openai::text-embedding-3-large", "key": "sk-proj-abc123..."}');
+```
+
+### Method 3: Using rembed_client_options
+The most flexible approach:
+
+```sql
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-client',
+   rembed_client_options(
+     'format', 'openai',
+     'model', 'text-embedding-3-small',
+     'key', 'sk-proj-abc123...'
+   )
+  );
+```
+
+### Method 4: Environment Variables (Still Supported)
+For production deployments, you can still use environment variables:
+
+```bash
+export OPENAI_API_KEY="sk-proj-abc123..."
+export GEMINI_API_KEY="AIza..."
+```
+
+Then register without keys in SQL:
+```sql
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-openai', 'openai::text-embedding-3-small');
+```
+
+## 🎯 Complete Examples
+
+### OpenAI with API Key
+```sql
+-- Simple format
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('openai-embed', 'openai:sk-proj-your-key-here');
+
+-- JSON format
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('openai-embed', '{"provider": "openai", "api_key": "sk-proj-your-key-here"}');
+
+-- Use it
+SELECT rembed('openai-embed', 'Hello, world!');
+```
+
+### Multiple Providers with Keys
+```sql
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  -- OpenAI
+  ('gpt-small', 'openai:sk-proj-abc123'),
+  ('gpt-large', '{"model": "openai::text-embedding-3-large", "key": "sk-proj-abc123"}'),
+
+  -- Gemini
+  ('gemini', 'gemini:AIzaSy...'),
+
+  -- Anthropic
+  ('claude', '{"provider": "anthropic", "api_key": "sk-ant-..."}'),
+
+  -- Local models (no key needed)
+  ('local-llama', 'ollama::llama2'),
+  ('local-nomic', 'ollama::nomic-embed-text');
+```
+
+### Dynamic Key Management
+```sql
+-- Create a table to store API keys
+CREATE TABLE api_keys (
+  provider TEXT PRIMARY KEY,
+  key TEXT NOT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Store keys securely
+INSERT INTO api_keys (provider, key) VALUES
+  ('openai', 'sk-proj-...'),
+  ('gemini', 'AIza...');
+
+-- Register clients using stored keys
+INSERT INTO temp.rembed_clients(name, options)
+SELECT
+  provider || '-client',
+  provider || ':' || key
+FROM api_keys;
+```
+
+## 🔒 Security Considerations
+
+### Development vs Production
+
+**Development** - API keys in SQL are convenient:
+```sql
+-- Quick testing with inline keys
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('test', 'openai:sk-test-key');
+```
+
+**Production** - Use environment variables:
+```bash
+# Set in environment
+export OPENAI_API_KEY="sk-proj-production-key"
+```
+
+```sql
+-- Reference without exposing key
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('prod', 'openai::text-embedding-3-small');
+```
+
+### Best Practices
+
+1. **Never commit API keys** to version control
+2. **Use environment variables** in production
+3. **Rotate keys regularly**
+4. **Use restricted keys** when possible (limited scope/permissions)
+5. **Store keys encrypted** if persisting in database
+
+## 🎨 Provider-Specific Formats
+
+| Provider | Simple Format | Environment Variable |
+|----------|--------------|---------------------|
+| OpenAI | `openai:sk-proj-...` | `OPENAI_API_KEY` |
+| Gemini | `gemini:AIza...` | `GEMINI_API_KEY` |
+| Anthropic | `anthropic:sk-ant-...` | `ANTHROPIC_API_KEY` |
+| Groq | `groq:gsk_...` | `GROQ_API_KEY` |
+| Cohere | `cohere:co-...` | `CO_API_KEY` |
+| DeepSeek | `deepseek:sk-...` | `DEEPSEEK_API_KEY` |
+| Mistral | `mistral:...` | `MISTRAL_API_KEY` |
+| Ollama | `ollama::model` | None (local) |
+
+## 🚀 Quick Start
+
+```sql
+-- Load the extension
+.load ./rembed0
+
+-- Register OpenAI with inline key (development)
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('embedder', 'openai:sk-proj-your-key-here');
+
+-- Generate embeddings
+SELECT length(rembed('embedder', 'Hello, world!'));
+
+-- Register multiple providers
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('fast', 'openai:sk-proj-key1'),
+  ('accurate', '{"model": "openai::text-embedding-3-large", "key": "sk-proj-key1"}'),
+  ('free', 'ollama::nomic-embed-text');
+
+-- Use different models
+SELECT rembed('fast', 'Quick embedding');
+SELECT rembed('accurate', 'Precise embedding');
+SELECT rembed('free', 'Local embedding');
+```
+
+## 🎭 Migration from Environment Variables
+
+If you're currently using environment variables and want to switch to SQL-based keys:
+
+```sql
+-- Before (requires OPENAI_API_KEY env var)
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-client', 'openai');
+
+-- After (self-contained)
+INSERT INTO temp.rembed_clients(name, options) VALUES
+  ('my-client', 'openai:sk-proj-your-key-here');
+```
+
+Both methods continue to work, giving you flexibility in deployment!