init: AI Grading Assistant MVP

database/schema.sql

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+CREATE TABLE IF NOT EXISTS exam (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  name TEXT NOT NULL,
+  course TEXT NOT NULL,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS question (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  exam_id INTEGER NOT NULL,
+  title TEXT NOT NULL,
+  max_score INTEGER NOT NULL DEFAULT 100,
+  rubric TEXT,
+  FOREIGN KEY (exam_id) REFERENCES exam(id)
+);
+
+CREATE TABLE IF NOT EXISTS history (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  question_id INTEGER DEFAULT 0,
+  image_path TEXT DEFAULT '',
+  student_answer TEXT DEFAULT '',
+  ai_score REAL DEFAULT 0,
+  teacher_score REAL DEFAULT 0,
+  confidence REAL DEFAULT 0,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);