Bound HTTP request timeouts and retry budget (#28)

* Bound HTTP request timeouts and retry budget

With no request timeout, Retry(total=30, backoff_factor=2,
backoff_max=60) let a single unreachable server hang the client for
around 26 minutes: backoff sleeps alone sum to that, and each of the
30 connection attempts could itself stall indefinitely without a
timeout. Measured a black-holed connect attempt taking 75s on this
machine with no timeout set, confirming the mechanism.

- Add HTTP_TIMEOUT (connect, read) and a tighter retry budget
  (HTTP_RETRY_TOTAL/BACKOFF_FACTOR/BACKOFF_MAX) in dsc/const.py,
  bounding the worst case to a few minutes.
- Pass timeout=HTTP_TIMEOUT on both SeafileClient HTTP calls (token
  fetch, remote library listing).
- Add NetworkError(DscError) so a network failure is caught by
  start.py's existing top-level DscError handling instead of
  producing a bare traceback.
- Add tests/test_http.py covering the retry policy values, timeout
  propagation, and error wrapping.
- Check off the corresponding IMPROVEMENTS.md items.

* Include URL and status code in NetworkError messages

Copilot review on PR #28 flagged that the non-200 path for remote
library listing raised NetworkError with only the raw response body,
losing which request failed and its status code. The token-fetch
error had the same gap. Both now include the URL and HTTP status.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

dsc/const.py

@@ -11,6 +11,16 @@ DAEMON_POLL_PERIOD = 1
 DAEMON_START_TIMEOUT = 120
 DAEMON_STOP_TIMEOUT = 8
 
+# Bounds for HTTP requests to the Seafile server. Without a request timeout,
+# a total=30/backoff_factor=2/backoff_max=60 retry policy was measured to
+# take about 26 minutes against an unreachable host before giving up, which
+# is indistinguishable from a hang. HTTP_TIMEOUT is (connect, read); the
+# retry budget below now bounds the worst case to a few minutes instead.
+HTTP_TIMEOUT = (10, 30)
+HTTP_RETRY_TOTAL = 5
+HTTP_RETRY_BACKOFF_FACTOR = 1
+HTTP_RETRY_BACKOFF_MAX = 30
+
 AVAILABLE_SEAFCLI_OPTIONS = {
     "delete_confirm_threshold",
     "disable_verify_certificate",