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/controller/groups/settings.py

@@ -2,12 +2,15 @@
 
 Exposes the "成员操作日志溯源" (member operation traceability) surface:
 
-* ``GET  /api/v1/settings/governance``            read level + template table
-* ``PUT  /api/v1/settings/governance``            change level / retention
-* ``GET  /api/v1/settings/operation-logs``        page through records
-* ``GET  /api/v1/settings/operation-logs/filters`` available filter values
-* ``POST /api/v1/settings/operation-logs/prune``  enforce retention now
-* ``DEL  /api/v1/settings/operation-logs``        clear records
+* ``GET /api/v1/settings/governance``              read level + template table
+* ``PUT /api/v1/settings/governance``              change level / retention
+* ``GET /api/v1/settings/operation-logs``          page through records
+* ``GET /api/v1/settings/operation-logs/filters``  available filter values
+* ``GET /api/v1/settings/operation-logs/export``   download the filtered CSV
+
+Operation records are append-only and there is no route to delete them on
+demand: retention is the only deletion path, and it runs automatically when
+the governance settings change and from the maintenance loop.
 
 Every route requires ``audit.view``, which is granted to the Workspace owner
 and admin only. Write routes additionally require an owning/admin role so a