Postgres real backend: psycopg adapter, schema, migration

Replace the PostgresDatabase skeleton with a real implementation:
psycopg (v3) first, psycopg2 second, lazy load, missing driver
still raises PostgresDriverMissingError with no silent SQLite
fallback. A sqlite3-style adapter (PgConnection/PgCursor/PgRow)
translates ? placeholders to %s outside string literals, maps
UNIQUE/FK violations, lock errors and connection failures to
explicit Postgres error types, and handles INSERT OR REPLACE,
PRAGMA table_info, AUTOINCREMENT and FTS5 refusal. POSTGRES_SCHEMA
covers all business tables with tasks result columns; server
init_db uses it under CLOUD_DB_BACKEND=postgres, SQLite path
unchanged. migrate_sqlite_to_postgres.py copies rows with
ON CONFLICT DO NOTHING and reseeds identity sequences.
Tests: cloud 239 (235 pass, 4 skipped without a real server),
host 27 pass; real-server tests skip unless CLOUD_TEST_PG_DSN
is set.

cloud/migrate_sqlite_to_postgres.py

@@ -0,0 +1,37 @@
+"""把 SQLite 控制面库迁移到 Postgres 的命令行入口。
+
+用法（Postgres 连接参数只走 CLOUD_DB_* 环境变量，本脚本
+不接受、不打印任何密码或密钥）：
+
+    CLOUD_DB_HOST=... CLOUD_DB_PORT=5432 CLOUD_DB_NAME=... \
+    CLOUD_DB_USER=... CLOUD_DB_PASSWORD=... \
+    python3 -m cloud.migrate_sqlite_to_postgres --sqlite <sqlite 文件>
+
+输出：每张表本次插入的行数（第二次跑应全为 0，即幂等，
+不重复插）。驱动缺失时明确报错，绝不静默回落到 SQLite；
+目标库须事先存在（本脚本不建库，建库见 cloud/README.md）。
+"""
+
+import argparse
+import sys
+
+from cloud.migrations import migrate_sqlite_to_postgres
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(
+        description="Migrate a SQLite control-plane DB to Postgres.")
+    parser.add_argument("--sqlite", required=True,
+                        help="source SQLite file path")
+    args = parser.parse_args(argv)
+    result = migrate_sqlite_to_postgres(args.sqlite)
+    total = 0
+    for table, count in result.items():
+        print("%s: %d" % (table, count))
+        total += count
+    print("total inserted: %d" % total)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())