fix(redaction): make URL-userinfo redaction linear, not quadratic

_REDACT_URL_USERINFO could start a match at every letter of a run of
scheme characters, and each attempt read to the end of the run looking
for `://`. On a long unbroken run of letters or digits (a hex digest, an
ID, part of a response body) that is quadratic: 1.6s for 20k characters.

The display service redacts every message, stack trace and context value
it publishes in the error snapshot, holding the aggregator lock, and
re.sub holds the GIL for the whole call, so one such exception stalled
every thread, render loop included (~0.5s measured for 20k chars of hex).
It also made test_snapshot_stays_small the slowest test in the suite by
far: 142s of a 383s run, 139s of it in this one regex.

A match may now only start where a run of scheme characters starts
(negative lookbehind). Leading digits and `+.-` are captured in group 1
so the substitution restores them, and the scheme still has to start
with a letter, so what gets redacted is unchanged: old and new output
were identical on 300k fuzzed inputs. 20k chars now take ~0.5ms, 200k
~6ms, and test_snapshot_stays_small takes 0.8s.

test/test_redaction.py pins the exact output for schemes that begin after
digits or `+.-`, and bounds 50k-character runs at 1s; against the old
pattern those timing tests fail at 3-11s each.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KMXdS2S4NXTJ8ET96GymhK

CHANGELOG.md

@@ -120,6 +120,12 @@ floor on the release that ships them):
   when the count is only known to the display service.
 - The Logs tab has a **Plugin errors** panel: per-plugin counts, repeating
   errors and a Clear button.
+- Redacting `user:password@` from URLs in exception text (`src/redaction.py`)
+  takes time proportional to the text, not its square. A plugin error quoting
+  a long unbroken run of letters or digits (a hex digest, an ID) used to stall
+  every thread of the display service for up to seconds each time the snapshot
+  was published: about 0.5s for 20k characters of hex. What gets redacted is
+  unchanged.
 
 ### Removed