feat(decrypt): 解密支持 SSE 实时进度

- 新增 /api/decrypt_stream（GET + SSE）：扫描 db_storage，逐库解密并推送 start/progress/complete/error

- 前端解密页优先使用 SSE 展示实时进度，不支持时回退到原 POST（无进度）

- 增加流式接口单测：验证事件序列与输出落盘

tests/test_decrypt_stream_sse.py

@@ -0,0 +1,91 @@
+import json
+import os
+import sys
+import unittest
+import importlib
+from pathlib import Path
+from tempfile import TemporaryDirectory
+
+
+ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(ROOT / "src"))
+
+
+class TestDecryptStreamSSE(unittest.TestCase):
+    def test_decrypt_stream_reports_progress(self):
+        from fastapi import FastAPI
+        from fastapi.testclient import TestClient
+
+        from wechat_decrypt_tool.wechat_decrypt import SQLITE_HEADER
+
+        with TemporaryDirectory() as td:
+            root = Path(td)
+
+            prev_data_dir = os.environ.get("WECHAT_TOOL_DATA_DIR")
+            prev_build_cache = os.environ.get("WECHAT_TOOL_BUILD_SESSION_LAST_MESSAGE")
+            try:
+                os.environ["WECHAT_TOOL_DATA_DIR"] = str(root)
+                os.environ["WECHAT_TOOL_BUILD_SESSION_LAST_MESSAGE"] = "0"
+
+                import wechat_decrypt_tool.app_paths as app_paths
+                import wechat_decrypt_tool.routers.decrypt as decrypt_router
+
+                importlib.reload(app_paths)
+                importlib.reload(decrypt_router)
+
+                db_storage = root / "xwechat_files" / "wxid_foo_bar" / "db_storage"
+                db_storage.mkdir(parents=True, exist_ok=True)
+
+                # Fake a decrypted sqlite db (>= 4096 bytes) so decryptor falls back to copy.
+                (db_storage / "MSG0.db").write_bytes(SQLITE_HEADER + b"\x00" * (4096 - len(SQLITE_HEADER)))
+
+                app = FastAPI()
+                app.include_router(decrypt_router.router)
+                client = TestClient(app)
+
+                events: list[dict] = []
+                with client.stream(
+                    "GET",
+                    "/api/decrypt_stream",
+                    params={"key": "00" * 32, "db_storage_path": str(db_storage)},
+                ) as resp:
+                    self.assertEqual(resp.status_code, 200)
+                    self.assertIn("text/event-stream", resp.headers.get("content-type", ""))
+
+                    for line in resp.iter_lines():
+                        if not line:
+                            continue
+                        if isinstance(line, bytes):
+                            line = line.decode("utf-8", errors="ignore")
+                        line = str(line)
+
+                        if line.startswith(":"):
+                            continue
+                        if not line.startswith("data: "):
+                            continue
+                        payload = json.loads(line[len("data: ") :])
+                        events.append(payload)
+                        if payload.get("type") in {"complete", "error"}:
+                            break
+
+                types = {e.get("type") for e in events}
+                self.assertIn("start", types)
+                self.assertIn("progress", types)
+                self.assertEqual(events[-1].get("type"), "complete")
+
+                out = root / "output" / "databases" / "wxid_foo" / "MSG0.db"
+                self.assertTrue(out.exists())
+            finally:
+                if prev_data_dir is None:
+                    os.environ.pop("WECHAT_TOOL_DATA_DIR", None)
+                else:
+                    os.environ["WECHAT_TOOL_DATA_DIR"] = prev_data_dir
+                if prev_build_cache is None:
+                    os.environ.pop("WECHAT_TOOL_BUILD_SESSION_LAST_MESSAGE", None)
+                else:
+                    os.environ["WECHAT_TOOL_BUILD_SESSION_LAST_MESSAGE"] = prev_build_cache
+
+
+if __name__ == "__main__":
+    unittest.main()
+