Bind sender identities, add operator read surface, real serving layer

AUDIT-WP-0004 T03, T05, T06.

T03 - the receiver accepted whatever tenant and source a caller sent as long
as it held the one shared token, despite WP-0003 recording tenant isolation as
delivered. audit_core.senders binds each credential to the sources and tenants
it may assert, driven by AUDIT_CORE_SENDERS rather than literals. Identities
hold a list of tokens so rotation publishes the replacement alongside the
incumbent and needs no delivery gap. Read is a separate privilege from write,
so a sender credential cannot read the audit trail back.

T05 - lookup by event id, lookup by correlation id, and a dead-letter view.
Rejections are recorded rather than silently dropped. An event rejected for
carrying secret-shaped material has its payload withheld: storing it would
write that material into the audit store, which is what the rejection exists
to prevent. Reason and payload hash are kept so it stays traceable.

Replay is deliberately not built here. Idempotent replay is a property of the
durable store and building it against SQLite would produce a second
implementation to throw away; it lands with the Postgres backend in
AUDIT-WP-0005-T01.

T06 - serving moves to waitress with configurable threads and channel timeout,
installed in the image via the serve extra. Without it the entrypoint falls
back to a threaded wsgiref server with a socket timeout and graceful shutdown
on SIGTERM, and logs a warning so a deployment cannot quietly land on the
fallback. Metric counters deferred to WP-0005-T03 to be designed against the
real scrape path.

Tests 36 -> 46, covering cross-tenant and cross-source refusal, token
rotation, read/write privilege separation, correlation lookup, and payload
withholding on secret rejection.

Remaining in WP-0004: T04 redaction policy, which needs a decision on whether
a secret-shaped field is a rejection or a redaction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Containerfile

@@ -3,7 +3,7 @@ RUN useradd --system --uid 10001 --create-home audit-core
 WORKDIR /app
 COPY pyproject.toml README.md LICENSE ./
 COPY audit_core ./audit_core
-RUN pip install --no-cache-dir .
+RUN pip install --no-cache-dir ".[serve]"
 USER 10001
 EXPOSE 8080
 CMD ["audit-core-ingest"]