feat: client log with request ids, joined to the server's

The browser now keeps its half of the one event shape logs.py defines:
lib/log.ts is a levelled, redacted, bounded (2000 events) in-memory ring
with the server's level names and severity numbers. Nothing leaves the
browser yet; sending is the consented diagnostics level (S5).

- lib/http.ts `send` is the one fetch: every request carries an
  X-Request-Id and becomes one http.request event. The server echoes the
  matched route template in X-Route (only for a matched route, so a raw
  path with a value in it is never sent back); without one the client
  templates the path itself. req, beacon, the upload XHR, the public
  watch page and the legal pages all go through it.
- Hooks: app start snapshot, uncaught errors and rejections, History
  push/replace/popstate (a scroll-only replace is not an event; parameter
  values only at DEBUG, typed search only at TRACE), notifications, the
  error dialog, the crash boundary, failed queries and mutations (key
  head only), and the messages socket (lifecycle, frame types at DEBUG).
- Redaction is one contract: backend/tests/fixtures/log_redaction_cases.json
  runs against both redact() implementations.
- Lint: fetch, XMLHttpRequest and WebSocket each belong to one logging
  module, console to none; a contract test proves the rules fire.

Fixes a socket takeover: the close of a socket released by the last
subscriber cleared the newer socket a quick re-subscribe had opened and
scheduled a third, leaving two live sockets that delivered every message
twice (measured in Chromium: 3 created / 2 live before, 2 / 1 after).

backend/tests/test_logs.py

@@ -11,6 +11,7 @@ Pure logic + an in-process ASGI app: no database.
 
 import json
 import logging
+from pathlib import Path
 
 import pytest
 from fastapi import FastAPI, Request
@@ -101,6 +102,18 @@ def test_an_email_in_an_attribute_value_is_scrubbed_too():
     assert logs.redact({"to": "a.b+tag@mail.example.org"})["to"] == logs.REDACTED_EMAIL
 
 
+# The SAME cases frontend/src/lib/log.redaction.test.ts runs against the browser's copy of redact():
+# the one thing that keeps the two logs' idea of "safe to keep" from drifting apart.
+_REDACTION_CASES = json.loads(
+    (Path(__file__).parent / "fixtures" / "log_redaction_cases.json").read_text(encoding="utf-8")
+)["cases"]
+
+
+@pytest.mark.parametrize("case", _REDACTION_CASES, ids=[c["name"] for c in _REDACTION_CASES])
+def test_the_shared_redaction_cases(case):
+    assert logs.redact(case["value"], key=case["key"]) == case["expected"]
+
+
 def test_redaction_runs_on_the_message_the_attrs_and_the_traceback(sink):
     log = logging.getLogger("siftlode.test")
     try:
@@ -263,6 +276,14 @@ def test_the_request_id_is_echoed_and_shared_by_every_line_of_that_request(sink)
     assert rid and sink.objects()[-1]["request_id"] == rid
 
 
+def test_the_route_template_goes_back_to_the_browser_but_never_a_raw_path(sink):
+    # The browser logs its own copy of each request, grouped by this template (frontend lib/http.ts).
+    client = _client()
+    assert client.get("/api/videos/SeCrEt-VaLuE").headers["X-Route"] == "/api/videos/{video_id}"
+    # Unmatched: the only template there is the raw path, which may carry a value — so none is sent.
+    assert "X-Route" not in client.get("/api/share/SeCrEt-VaLuE").headers
+
+
 def test_a_client_supplied_id_is_reused_only_when_it_is_well_formed(sink):
     client = _client()
     good = client.get("/api/videos/x", headers={"X-Request-Id": "web-0123456789ab"})