Replace deprecated raven client with sentry-sdk in SentryReporter

Start with backlash/tracing/reporters/sentry.py and review SentryReporter and RavenNotAvailable, then inspect backlash/frtools.py for the backlash_event flag. Check the deprecated re-export in backlash/trace_errors/sentry.py and the Sentry section of README.rst. Done means crash and slow-request reports use sentry-sdk, the raven import is gone, the flag distinction remains, and the optional dependency is documented.

Goal

Port SentryReporter from the retired raven client to the current
sentry-sdk.

Done when: crash reports and slow-request stack dumps reach Sentry through
sentry-sdk, the raven import is gone, and the optional dependency is
documented for users who want Sentry reporting.

Points that need attention during the port:

• raven.base.Client(sentry_dsn) has no direct equivalent; sentry_sdk uses
  sentry_sdk.init(dsn=...) plus a Client/Scope model.
• client.captureException(exc_info=..., data=...) and
  client.captureMessage(..., stack=...) map to capture_exception /
  capture_event with a different payload shape.
• The sentry.interfaces.Http dict is assembled by hand from
  raven.utils.wsgi helpers (get_current_url, get_headers, get_environ),
  which do not exist in sentry-sdk; the WSGI request data has to be built
  another way.
• The backlash-specific backlash_event flag distinguishes a slow-request
  stack dump from a real crash, and that distinction should survive the port.

Why

raven was last released in December 2018, is officially retired in favour of
sentry-sdk, and advertises support only up to Python 3.6. backlash now
supports Python 3.9 through 3.14, so the Sentry reporting feature is
effectively unavailable to current users: nobody can reasonably install the
client it requires.

The import is optional and wrapped in try/except ImportError, so the failure
is silent - SentryReporter simply raises RavenNotAvailable at construction
time. Documented functionality therefore looks supported while being
unreachable in practice.

References

• backlash/tracing/reporters/sentry.py (SentryReporter, RavenNotAvailable)
• backlash/trace_errors/sentry.py is the deprecated shim re-exporting it
• README.rst (Sentry reporting section)
• backlash/frtools.py (DumpThread) is the source of the backlash_event flag
• sentry-sdk migration guide (raven -> sentry-sdk)