add pipeline id to sql conv store

scripts/init_database/create_conv_store.sql

@@ -2,6 +2,7 @@
 CREATE TABLE IF NOT EXISTS messages (
     id BIGSERIAL PRIMARY KEY,
     conversation_id TEXT NOT NULL,
+    pipeline_id TEXT,
     message_type VARCHAR(10) NOT NULL CHECK (message_type IN ('human', 'ai', 'tool')),
     content TEXT NOT NULL,
     sequence_number INTEGER NOT NULL CHECK (sequence_number >= 0),
@@ -10,3 +11,6 @@ CREATE TABLE IF NOT EXISTS messages (
 
 -- Index for fast retrieval of all messages in a conversation (in order)
 CREATE INDEX IF NOT EXISTS idx_messages_conversation ON messages (conversation_id, sequence_number);
+
+-- Index for fast lookup by pipeline_id
+CREATE INDEX IF NOT EXISTS idx_messages_pipeline ON messages (pipeline_id);