Addresses review feedback to separate HTTP API definitions from general modules. The api module should only contain HTTP route handlers, relying on other modules for shared types.

Changes

• New crate::ai module: Moved provider configuration types (AuthMethod, ProviderConfig, ProviderConfigBuilder) from api/ai/provider.rs to ai/provider.rs
• Flattened API module: Converted api/ai/ directory to single api/ai.rs file that imports from crate::ai

// Before: api/ai/mod.rs
mod provider;
pub use provider::{AuthMethod, ProviderConfig, ProviderConfigBuilder};

// After: api/ai.rs
use crate::ai::{AuthMethod, ProviderConfig, ProviderConfigBuilder};

This follows the established pattern where API modules are flat and delegate to general-purpose modules for shared functionality.

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