feat(dimentorin): AI agent RAG (embedding materi -> Qdrant + chat via 9router)

- ENV: AI_LLM_BASE_URL/API_KEY/MODEL, AI_EMBEDDING_MODEL (gemini-embedding-001), QDRANT_URL
- ai_agent module: chunking materi, embed_text + chat_completion (9router, SSE parse), Qdrant repo (dimentorin_materi collection, 3072d cosine)
- routes: POST /ai/chat (RAG answer + sources), POST /ai/materials/{id}/index, POST /ai/reindex
- e2e verified: reindex 3 chunks; chat 'ownership' -> materi Rust paling relevan 0.88; chat 'endpoint axum' -> materi Axum 0.82; jawaban gronding konteks

imphnen-dimentorin/src/ai_agent/domain/mod.rs

@@ -0,0 +1,9 @@
+pub mod chat_types;
+pub mod rag_document;
+pub mod repository;
+pub mod service;
+
+pub use chat_types::{ChatRequest, ChatResponse, RagSource};
+pub use rag_document::RagDocument;
+pub use repository::RagRepository;
+pub use service::RagService;