add better indexes for symbol rank

Author: DolceTriade

backend/migrations/0014_symbol_search_indexes.sql

@@ -0,0 +1,9 @@
+-- Speed up symbol-search reranking queries by indexing content-hash scoped symbol lookups
+-- and definition-only symbol reference joins.
+
+CREATE INDEX IF NOT EXISTS idx_symbol_references_definition_symbol_id
+    ON symbol_references (symbol_id)
+    WHERE kind = 'definition';
+
+CREATE INDEX IF NOT EXISTS idx_symbols_content_hash_name_lc_id
+    ON symbols (content_hash, name_lc, id);