refactor: reorganize folder structure for better maintainability

Changes:
- Move migrations/ under open_notebook/database/migrations/
- Extract AI models to open_notebook/ai/ (Model, ModelManager, provision)
- Extract podcasts to open_notebook/podcasts/ (EpisodeProfile, SpeakerProfile, PodcastEpisode)
- Reorganize prompts to mirror graphs structure (chat/, source_chat/)

This improves code organization by:
- Consolidating database concerns (migrations now with database code)
- Separating AI infrastructure from domain entities
- Isolating podcast feature into its own module
- Creating consistent prompt/graph naming conventions

All 52 tests pass.

open_notebook/database/migrations/3_down.surrealql

@@ -0,0 +1,110 @@
+REMOVE TABLE IF EXISTS chat_session;
+
+REMOVE TABLE IF EXISTS refers_to;
+
+
+REMOVE FUNCTION fn::vector_search;
+
+
+DEFINE FUNCTION IF NOT EXISTS fn::vector_search($query: array<float>, $match_count: int, $sources:bool, $show_notes:bool) {
+   
+    let $source_embedding_search = 
+         IF $sources {(
+            SELECT source as item_id, content, vector::similarity::cosine(embedding, $query) as similarity
+            FROM source_embedding LIMIT $match_count)}
+        ELSE { [] };
+
+    
+    let $source_insight_search = 
+         IF $sources {(
+             SELECT source as item_id, content, vector::similarity::cosine(embedding, $query) as similarity
+                FROM source_insight LIMIT $match_count)}
+        ELSE { [] };
+
+    
+     let $note_content_search = 
+         IF $show_notes {(
+                SELECT id as item_id, content, vector::similarity::cosine(embedding, $query) as similarity
+                FROM note LIMIT $match_count)}
+
+        ELSE { [] };
+
+    let $source_chunk_results = array::union($source_embedding_search, $source_insight_search);
+    
+    let $source_results = array::union($source_chunk_results, $source_insight_search);
+
+    let $note_results = $note_content_search;
+    let $final_results = array::union($source_results, $note_results );
+
+    RETURN (SELECT item_id, math::max(similarity) as similarity from $final_results
+        group by item_id ORDER BY similarity DESC LIMIT $match_count);
+    
+    
+};
+
+REMOVE FUNCTION fn::text_search;
+
+
+DEFINE FUNCTION IF NOT EXISTS fn::text_search($query_text: string, $match_count: int, $sources:bool, $show_notes:bool) {
+  
+    let $source_title_search = 
+        IF $sources {(
+            SELECT id as item_id, math::max(search::score(1)) AS relevance
+            FROM source
+            WHERE title @1@ $query_text
+            GROUP BY item_id)}
+        ELSE { [] };
+    
+    let $source_embedding_search = 
+         IF $sources {(
+             SELECT source as item_id, math::max(search::score(1)) AS relevance
+            FROM source_embedding
+            WHERE content @1@ $query_text
+            GROUP BY item_id)}
+        ELSE { [] };
+
+    let $source_full_search = 
+         IF $sources {(
+            SELECT source as item_id, math::max(search::score(1)) AS relevance
+            FROM source
+            WHERE full_text @1@ $query_text
+            GROUP BY item_id)}
+        ELSE { [] };
+    
+    let $source_insight_search = 
+         IF $sources {(
+             SELECT source as item_id, math::max(search::score(1)) AS relevance
+            FROM source_insight
+            WHERE content @1@ $query_text
+            GROUP BY item_id)}
+        ELSE { [] };
+
+    let $note_title_search = 
+         IF $show_notes {(
+             SELECT id as item_id, math::max(search::score(1)) AS relevance
+            FROM note
+            WHERE title @1@ $query_text
+            GROUP BY item_id)}
+        ELSE { [] };
+
+     let $note_content_search = 
+         IF $show_notes {(
+             SELECT id as item_id, math::max(search::score(1)) AS relevance
+            FROM note
+            WHERE content @1@ $query_text
+            GROUP BY item_id)}
+        ELSE { [] };
+
+    let $source_chunk_results = array::union($source_embedding_search, $source_full_search);
+    
+    let $source_asset_results = array::union($source_title_search, $source_insight_search);
+
+    let $source_results = array::union($source_chunk_results, $source_asset_results );
+    let $note_results = array::union($note_title_search, $note_content_search );
+    let $final_results = array::union($source_results, $note_results );
+
+    RETURN (SELECT item_id, math::max(relevance) as relevance from $final_results
+        group by item_id ORDER BY relevance DESC LIMIT $match_count);
+    
+    
+};