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/4_down.surrealql

@@ -0,0 +1,139 @@
+
+REMOVE FUNCTION IF EXISTS fn::vector_search;
+
+DEFINE FUNCTION IF NOT EXISTS fn::vector_search($query: array<float>, $match_count: int, $sources: bool, $show_notes: bool, $min_similarity: float) {
+    let $source_embedding_search = 
+        IF $sources {(
+            SELECT 
+                id,
+                source.title as title,
+                content,
+                source.id as parent_id,
+                vector::similarity::cosine(embedding, $query) as similarity
+            FROM source_embedding 
+            WHERE vector::similarity::cosine(embedding, $query) >= $min_similarity
+            ORDER BY similarity DESC
+            LIMIT $match_count
+        )}
+        ELSE { [] };
+
+    let $source_insight_search = 
+        IF $sources {(
+            SELECT 
+                id,
+                insight_type + ' - ' + source.title as title,
+                content,
+                source.id as parent_id,
+                vector::similarity::cosine(embedding, $query) as similarity
+            FROM source_insight
+            WHERE vector::similarity::cosine(embedding, $query) >= $min_similarity
+            ORDER BY similarity DESC
+            LIMIT $match_count
+        )}
+        ELSE { [] };
+
+
+    let $note_content_search = 
+        IF $show_notes {(
+            SELECT 
+                id,
+                title,
+                content,
+                id as parent_id,
+                vector::similarity::cosine(embedding, $query) as similarity
+            FROM note
+            WHERE vector::similarity::cosine(embedding, $query) >= $min_similarity
+            ORDER BY similarity DESC
+            LIMIT $match_count
+        )}
+        ELSE { [] };
+
+
+    let $all_results = array::union(
+        array::union($source_embedding_search, $source_insight_search),
+        $note_content_search
+    );
+
+
+    RETURN (
+        SELECT 
+            id, title, content, parent_id,
+            math::max(similarity) as similarity
+        FROM $all_results
+        GROUP BY id
+        ORDER BY similarity DESC
+        LIMIT $match_count
+    );
+};
+
+
+REMOVE FUNCTION IF EXISTS fn::text_search;
+
+
+DEFINE FUNCTION IF NOT EXISTS fn::text_search($query_text: string, $match_count: int, $sources:bool, $show_notes:bool) {
+  
+    let $source_title_search = 
+        IF $sources {(
+            SELECT id, title, 
+            search::highlight('`', '`', 1) as content,
+            id as parent_id,
+            math::max(search::score(1)) AS relevance
+            FROM source
+            WHERE title @1@ $query_text
+            GROUP BY id)}
+        ELSE { [] };
+    
+    let $source_embedding_search = 
+         IF $sources {(
+            SELECT id as id, source.title as title, search::highlight('`', '`', 1) as content, source.id as parent_id, math::max(search::score(1)) AS relevance
+            FROM source_embedding
+            WHERE content @1@ $query_text
+            GROUP BY id)}
+        ELSE { [] };
+
+    let $source_full_search = 
+         IF $sources {(
+            SELECT source.id as id, source.title as title, search::highlight('`', '`', 1) as content, source.id as parent_id, math::max(search::score(1)) AS relevance
+            FROM source
+            WHERE full_text @1@ $query_text
+            GROUP BY id)}
+        ELSE { [] };
+    
+    let $source_insight_search = 
+         IF $sources {(
+             SELECT id, insight_type + " - " + source.title as title, search::highlight('`', '`', 1) as content, source.id as parent_id,  math::max(search::score(1)) AS relevance
+            FROM source_insight
+            WHERE content @1@ $query_text
+            GROUP BY id)}
+        ELSE { [] };
+
+    let $note_title_search = 
+         IF $show_notes {(
+             SELECT id, title, search::highlight('`', '`', 1) as content,  id as parent_id, math::max(search::score(1)) AS relevance
+            FROM note
+            WHERE title @1@ $query_text
+            GROUP BY id)}
+        ELSE { [] };
+
+     let $note_content_search = 
+         IF $show_notes {(
+             SELECT id, title, search::highlight('`', '`', 1) as content,  id as parent_id, math::max(search::score(1)) AS relevance
+            FROM note
+            WHERE content @1@ $query_text
+            GROUP BY 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 id, title, content, parent_id, math::max(relevance) as relevance from $final_results
+        where id is not None        
+group by id, title, content, parent_id ORDER BY relevance DESC LIMIT $match_count);
+    
+    
+};