Add the PostgreSQL audit backend and a shared conformance suite

AUDIT-WP-0005-T01, built and verified against PostgreSQL 16 locally in
Docker; the Railiance cluster was not needed.

tests/test_backend_conformance.py is one suite run against every backend, so
"the Postgres backend is done" means it satisfies the same contract SQLite
already does rather than having its own green tests. It skips cleanly with no
server reachable; make pg-test-up and make test-pg run it. Suite 50 -> 71.

RetentionPolicy declares immutable=True and earns it: migration 0002 installs
a trigger rejecting UPDATE and DELETE on the events table, so a leaked runtime
credential can append but cannot rewrite or erase the trail. That materially
narrows the residual risk ADR-0001 section 5 called out. tamper_evidence stays
False because nothing here would prove a database owner had dropped the
trigger - hash-chaining or external anchoring would be needed and is not
implemented.

Idempotency is one statement (INSERT ... ON CONFLICT DO NOTHING RETURNING),
verified to behave identically to the SQLite backend under 12 concurrent
submissions of the same event. Migrations are ordered, recorded and
idempotent. Replay reconciles rather than duplicating - the piece deferred out
of WP-0004-T05 - and is tested to leave exactly one custody record.

Backend selection is by AUDIT_CORE_DATABASE_URL; the SQLite fallback logs a
warning so a deployment that lost its URL is visible rather than quietly
running on the wrong store.

Also fixed: ingestion had no __main__ guard, so python -m audit_core.ingestion
silently did nothing. Found during end-to-end smoke.

Counting semantics documented: occurrences counts transmissions, not stored
events, so a retry of a secret-shaped field increments it again. That is the
sender behaviour being optimized away.

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

docs/audit-backend-contract.md

@@ -296,6 +296,12 @@ Read them at `GET /v1/secret-findings` (requires the read privilege):
    "first_seen": "...", "last_seen": "..."}]}
 ```
 
+`occurrences` counts **transmissions, not stored events**: a retry resubmitting
+the same secret-shaped field increments it again, even though the event
+reconciles as a duplicate and produces no second custody record. That is
+deliberate — the number measures how often the sender emitted the field, which
+is the behaviour being optimized away.
+
 `persisted` distinguishes a field that reached the stored record from one that
 sat elsewhere in the envelope and was dropped by normalization anyway. A
 healthy sender trends to zero occurrences; a non-empty list is a backlog item