fix(operation-trace): repair export URL, drop dead action, prune unused keys

- Export download failed because the URL builder appended the path to a
  "/" base, producing a protocol-relative "//api/..." URL that the browser
  read as host "api"; the request never reached the backend. Resolve the
  base to the current origin, mirroring the other URL builders.
- Remove the orphan ``clear_prune`` action: the branch has no prune/clear
  route and no route rule maps to it, so it could never be produced. Its
  i18n key is removed too.
- Remove ``tamperedCount``, now unreferenced after the verification badge
  was split into integrity/chain counters. The operationTrace catalogue is
  now free of unproduced keys.
- Correct the settings controller docstring: only governance, operation
  logs, filters and export routes exist; there is no on-demand delete.

Verified: no producer for clear_prune, classify() can no longer emit it,
operationTrace orphan scan returns none, tsc/check-i18n/prettier/py_compile
all pass.

src/langbot/pkg/api/http/service/settings.py

@@ -212,12 +212,6 @@ ACTION_RULE_TABLE: typing.Final[tuple[ActionRule, ...]] = (
         bucket='audit',
         resource_type='operation_log',
     ),
-    ActionRule(
-        action='clear_prune',
-        category='audit',
-        bucket='write',
-        resource_type='operation_log',
-    ),
     ActionRule(
         action='settings_update',
         category='settings',