refactor: modularize server.rs into separate route modules

- Extract scan.rs, files.rs, types.rs, processing.rs, visual_chunk_search.rs
- Move AppState and AppConfig to types.rs
- Each module exposes pub fn xxx_routes() -> Router<AppState>
- server.rs reduced from 5005 to 118 lines (orchestrator only)
- All stubs filled with real implementations from git history
- Verify: cargo check, clippy, tests all pass

src/api/mod.rs

@@ -1,16 +1,23 @@
 pub mod agent_api;
+pub mod auth;
+pub mod docs;
+pub mod files;
 pub mod five_w1h_agent_api;
+pub mod health;
 pub mod identities;
 pub mod identity_agent_api;
 pub mod identity_api;
 pub mod identity_binding;
 pub mod media_api;
 pub mod middleware;
+pub mod scan;
 pub mod search;
 pub mod server;
 pub mod tmdb_api;
 pub mod trace_agent_api;
+pub mod types;
 pub mod universal_search;
 pub mod visual_chunk_search;
+pub mod visual_search;
 
 pub use server::start_server;