refactor(data): baseline 迁移改为完整幂等建库脚本，放宽 checksum 校验

- baseline migration 从占位 SELECT 1 替换为完整的幂等 DDL，支持全新数据库从零建库
- 迁移校验从 checksum 严格报错改为 warn-only，仅按版本匹配
- 移除过时的 schema 导出文件（README/TSV）
- Dockerfile 移除 :nonroot 标签，docker-compose 移除 user 指令，统一以默认用户运行

crates/aether-data/src/migrate.rs

@@ -137,6 +137,11 @@ fn validate_applied_migrations(
         }
     }
 
+    // Checksum drift is reported as a warning only. Strict enforcement makes
+    // harmless edits (comment tweaks, whitespace, metadata fixes) impossible
+    // without also touching every environment's migration history table. We
+    // match by version alone — sqlx still skips already-applied migrations so
+    // edited files will not re-run, they are merely allowed to exist.
     for migration in MIGRATOR
         .iter()
         .filter(|migration| migration.migration_type.is_up_migration())
@@ -146,12 +151,11 @@ fn validate_applied_migrations(
             .find(|applied_migration| applied_migration.version == migration.version)
         {
             if migration.checksum != applied_migration.checksum {
-                error!(
+                warn!(
                     version = migration.version,
                     description = %migration.description,
-                    "database migration checksum mismatch detected"
+                    "database migration checksum mismatch (ignored: version-only validation)"
                 );
-                return Err(MigrateError::VersionMismatch(migration.version));
             }
         }
     }