Added code for backend glue

backend/migrations/0001_init.sql

@@ -0,0 +1,40 @@
+-- users (minimal for v0)
+CREATE TABLE IF NOT EXISTS users (
+  id           INTEGER PRIMARY KEY AUTOINCREMENT,
+  email        VARCHAR(255) UNIQUE,
+  pass_bcrypt  BLOB,
+  created_at   TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- conversations
+CREATE TABLE IF NOT EXISTS conversations (
+  id           INTEGER PRIMARY KEY AUTOINCREMENT,
+  owner_id     INTEGER NOT NULL,
+  title        VARCHAR(255),
+  created_at   TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- nodes
+CREATE TABLE IF NOT EXISTS nodes (
+  id              INTEGER PRIMARY KEY AUTOINCREMENT,
+  conversation_id INTEGER NOT NULL,
+  author_kind     ENUM('user','assistant') NOT NULL,
+  content         TEXT NOT NULL,
+  created_at      TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+-- edges (DAG)
+CREATE TABLE IF NOT EXISTS edges (
+  parent_id    INTEGER NOT NULL,
+  child_id     INTEGER NOT NULL,
+  PRIMARY KEY(parent_id, child_id)
+);
+
+-- branches (named pointers)
+CREATE TABLE IF NOT EXISTS branches (
+  id              INTEGER PRIMARY KEY AUTOINCREMENT,
+  conversation_id INTEGER NOT NULL,
+  name            VARCHAR(128) NOT NULL,
+  head_node_id    INTEGER NOT NULL,
+  UNIQUE (conversation_id, name)
+);