feat(db): SQLite 连接与迁移框架（embed 迁移/事务校验/PG 预留 M4）

internal/db/migrations/0001_init.up.sql

@@ -0,0 +1,16 @@
+-- 0001_init: M0 migration-framework placeholder.
+--
+-- Creates a small, harmless key/value table so the migration plumbing
+-- (embed -> load -> apply in a transaction -> record in schema_migrations)
+-- can be exercised end to end. Real tables (messages, memories, sessions,
+-- personas, ...) are added as their own timestamped migrations in later
+-- milestones; this placeholder is NOT a domain table.
+--
+-- Note: key is the primary key on purpose — this is a key/value metadata
+-- table, so it intentionally deviates from the "id primary key" convention
+-- that applies to domain tables.
+CREATE TABLE IF NOT EXISTS app_meta (
+    key        TEXT PRIMARY KEY,
+    value      TEXT NOT NULL,
+    updated_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now'))
+);