feat: Add initial PostgreSQL migration, refactor database connection to k_core, and simplify session store setup.

migrations_sqlite/20240101000000_init_users.sql

@@ -0,0 +1,11 @@
+-- Create users table
+CREATE TABLE IF NOT EXISTS users (
+    id TEXT PRIMARY KEY NOT NULL,
+    subject TEXT NOT NULL,
+    email TEXT NOT NULL,
+    password_hash TEXT,
+    created_at TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_users_subject ON users(subject);
+CREATE UNIQUE INDEX IF NOT EXISTS idx_users_email ON users(email);