feat(knowledge): 实现 SQLite/FTS5 数据层与 Markdown Block 解析

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/database/migrations.py

@@ -0,0 +1,77 @@
+"""轻量 schema 迁移。
+
+约定：MIGRATIONS 列表按版本号顺序排列，只增不改。每一条是一个完整的 SQL 脚本，
+执行后写入 schema_migrations 记录版本。修改 Schema 时在末尾追加新脚本，禁止删旧脚本或
+依赖运行时自动删表重建（团队约定）。
+"""
+
+from datetime import datetime, timezone
+
+from app.constants import EMBEDDING_DIM
+
+# 每个元素对应一个版本（下标 + 1）。vec0 建表需要本连接已加载 sqlite-vec 扩展，
+# 由 db.connect() 在调用 migrate 之前完成。
+MIGRATIONS: list[str] = [
+    # v1: 笔记元数据 + Block + FTS5 全文索引 + 向量表 + 索引元信息
+    f"""
+    CREATE TABLE IF NOT EXISTS notes (
+        note_id     TEXT PRIMARY KEY,
+        title       TEXT NOT NULL,
+        file_path   TEXT NOT NULL UNIQUE,
+        folder      TEXT NOT NULL DEFAULT '',
+        tags        TEXT NOT NULL DEFAULT '[]',
+        created_at  TEXT NOT NULL,
+        updated_at  TEXT NOT NULL
+    );
+
+    CREATE TABLE IF NOT EXISTS blocks (
+        block_id     TEXT PRIMARY KEY,
+        note_id      TEXT NOT NULL REFERENCES notes(note_id) ON DELETE CASCADE,
+        heading_path TEXT NOT NULL DEFAULT '[]',
+        start_offset INTEGER NOT NULL,
+        end_offset   INTEGER NOT NULL,
+        content      TEXT NOT NULL,
+        content_hash TEXT NOT NULL,
+        token_count  INTEGER NOT NULL,
+        position     INTEGER NOT NULL
+    );
+    CREATE INDEX IF NOT EXISTS idx_blocks_note ON blocks(note_id, position);
+
+    CREATE VIRTUAL TABLE IF NOT EXISTS blocks_fts USING fts5(
+        block_id UNINDEXED,
+        note_id UNINDEXED,
+        heading_path,
+        content,
+        tokenize = 'unicode61'
+    );
+
+    CREATE VIRTUAL TABLE IF NOT EXISTS vec_blocks USING vec0(
+        block_id  TEXT PRIMARY KEY,
+        embedding float[{EMBEDDING_DIM}]
+    );
+
+    CREATE TABLE IF NOT EXISTS index_meta (
+        key   TEXT PRIMARY KEY,
+        value TEXT NOT NULL
+    );
+    """,
+]
+
+
+def migrate(conn) -> None:
+    """把尚未应用的迁移脚本按序应用到给定连接。"""
+    conn.execute(
+        "CREATE TABLE IF NOT EXISTS schema_migrations "
+        "(version INTEGER PRIMARY KEY, applied_at TEXT NOT NULL)"
+    )
+    applied = {row["version"] for row in conn.execute("SELECT version FROM schema_migrations")}
+
+    for idx, script in enumerate(MIGRATIONS, start=1):
+        if idx in applied:
+            continue
+        conn.executescript(script)
+        conn.execute(
+            "INSERT INTO schema_migrations (version, applied_at) VALUES (?, ?)",
+            (idx, datetime.now(timezone.utc).isoformat()),
+        )
+        conn.commit()