Authenticate from mounted secrets and stop passing the password to seaf-cli

The password could only be given through the PASSWORD variable, so Docker
Compose and Swarm secrets were unusable and the password sat in the container
environment.

PASSWORD_FILE and TOKEN_FILE read a secret from a mounted file. Only trailing
newlines are stripped, since secret files are usually written with one and a
password that silently carries it just fails to authenticate; everything else,
including surrounding spaces, belongs to the secret. Setting both a variable
and a file for one secret is rejected rather than resolved by precedence,
because the two are meant to be the same value and quietly picking one hides
the mistake until authentication fails.

The installed seaf-cli supports token authentication and ignores the password
when a token is given, so sync now passes -T instead of -p. The account
password no longer appears in any command line. The token does, because
seaf-cli accepts credentials only as arguments, but a token can be revoked on
the server. This is documented in the README.

Both secrets are masked before a command is logged.

Tests cover newline handling, empty and missing files, conflicting sources,
and that neither secret reaches the logs. Requests are now blocked in tests:
the client retries with backoff, so a test that reached the network appeared
to hang instead of failing.

IMPROVEMENTS.md

@@ -60,13 +60,21 @@ Acceptance criteria:
 
 ### Support secrets safely
 
-- [ ] Implement `PASSWORD_FILE` for Docker Compose and Swarm secrets.
-- [ ] Add token-based authentication and `TOKEN_FILE` if supported by the
-  installed Seafile CLI.
-- [ ] Define precedence and reject conflicting secret sources.
-- [ ] Ensure secrets are absent from application logs.
-- [ ] Document any upstream limitation that exposes a secret in process
-  arguments.
+- [x] Implement `PASSWORD_FILE` for Docker Compose and Swarm secrets. Only a
+  trailing newline is stripped, so a secret keeps any surrounding spaces.
+- [x] Add token-based authentication and `TOKEN_FILE` if supported by the
+  installed Seafile CLI. It is: `seaf-cli sync` takes `-T`, and ignores the
+  password when a token is given.
+- [x] Define precedence and reject conflicting secret sources. Two sources for
+  one secret (`PASSWORD` with `PASSWORD_FILE`) are rejected; a token takes
+  precedence over a password, as it does in `seaf-cli` itself.
+- [x] Ensure secrets are absent from application logs. Both the password and
+  the token are masked before a command is logged.
+- [x] Document any upstream limitation that exposes a secret in process
+  arguments. `seaf-cli` accepts credentials only as arguments, so a secret is
+  visible to any process in the container. The account password no longer goes
+  there: the client authenticates with an API token, which can be revoked.
+  See the README.
 
 Acceptance criteria: