feat: make bootstrapper initialiser timeouts configurable

DefaultFlowStart and WorkspaceInit hardcoded the request timeouts for
their flow-svc and IAM calls, leaving operators no way to tune them for
high-latency environments (#874).

Expose them as constructor parameters threaded through the existing
initialiser `params:` mechanism, defaulting to the current values so
behaviour is unchanged unless explicitly overridden:

- DefaultFlowStart: list_timeout=10 (list-flows), start_timeout=30 (start-flow)
- WorkspaceInit: iam_timeout=10 (create-workspace)

Add unit tests for the defaults, override storage, and that configured
values reach the underlying request calls.

tests/unit/test_bootstrap/test_workspace_init.py

@@ -0,0 +1,13 @@
+"""Unit tests for trustgraph.bootstrap.initialisers.WorkspaceInit."""
+
+from trustgraph.bootstrap.initialisers.workspace_init import WorkspaceInit
+
+
+def test_default_iam_timeout():
+    init = WorkspaceInit()
+    assert init.iam_timeout == 10
+
+
+def test_iam_timeout_override_is_stored():
+    init = WorkspaceInit(iam_timeout=42)
+    assert init.iam_timeout == 42