Run a container smoke test against the real seafile daemon

Startup, shutdown and secret handling were only ever checked by hand, and the
unit tests cannot check them: they stub seaf-cli, so nothing exercised the
daemon that actually ships. The GitHub runner is x86_64, which matches the
AppImage, so the image build job can run the real thing.

tests/smoke.sh starts and stops the real daemon, sends SIGTERM through docker
stop and asserts the container exits 0 within the grace period with the daemon
shut down, checks that a configuration error exits 2 naming the variable, that
a secret mounted under /run/secrets is read and stays out of the environment,
and that a default container gets exactly the documented defaults.

The pinned client version is exposed as SEAFILE_CLI_VERSION in the image,
since seaf-cli has no --version flag and there is no package manager entry
for an AppImage.

IMPROVEMENTS.md

@@ -134,10 +134,12 @@ Acceptance criteria:
   parsing, retries, and status parsing are not.
 - [ ] Add regression tests for externally reported password and deletion
   threshold issues.
-- [ ] Add a container smoke test for imports, installed CLI version, startup,
-  and shutdown using controlled fakes. Graceful shutdown was verified by hand
-  in the built image (`docker stop` returns immediately with exit code 0
-  instead of being killed after 10 seconds); it is not yet automated in CI.
+- [x] Add a container smoke test for imports, installed CLI version, startup,
+  and shutdown using controlled fakes. See `tests/smoke.sh`, run by the image
+  build job. It exercises the real `seaf-cli` and `seaf-daemon`, which the unit
+  tests cannot: the runner is x86_64, matching the AppImage. The pinned client
+  version is exposed as `SEAFILE_CLI_VERSION` in the image so it can be
+  asserted at runtime.
 - [x] Keep image construction as a separate CI job so packaging failures are
   distinguishable from application failures. The `unit-test` job does not
   depend on the image build.
@@ -160,7 +162,11 @@ Acceptance criteria:
 ### Networking and protocol behavior
 
 - [ ] Add explicit connect and read timeouts to all HTTP requests.
-- [ ] Define a bounded retry budget with backoff and jitter.
+- [ ] Define a bounded retry budget with backoff and jitter. Measured: the
+  current `Retry(total=30, backoff_factor=2, backoff_max=60)` with no request
+  timeout waits about 26 minutes on a single unreachable server before giving
+  up, which is indistinguishable from a hang. Observed while writing the smoke
+  test, when a probe pointed at an unroutable host.
 - [ ] Handle authentication, rate limits, malformed responses, and retryable
   server errors separately.
 - [ ] Make TLS configuration consistent between Python requests and Seafile.