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/9.surrealql

@@ -0,0 +1,66 @@
+
+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 
+                source.id as id,
+                source.title as title,
+                content,
+                source.id as parent_id,
+                vector::similarity::cosine(embedding, $query) as similarity
+            FROM source_embedding 
+            WHERE embedding != none and array::len(embedding)=array::len($query) AND
+                 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 OR '') as title,
+                content,
+                source.id as parent_id,
+                vector::similarity::cosine(embedding, $query) as similarity
+            FROM source_insight
+             WHERE embedding != none and array::len(embedding)=array::len($query) AND
+            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, $query) as similarity
+            FROM note
+            WHERE embedding != none and array::len(embedding)=array::len($query) AND
+            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, parent_id, title, math::max(similarity) as similarity,
+    array::flatten(content) as matches
+    from $all_results where id is not None
+    group by id, parent_id, title ORDER BY similarity DESC LIMIT $match_count);
+
+};