Accept a brokered libpq environment as connection information

AUDIT-WP-0005-T02. The rapp-postgres credential playbook has the
railiance-platform broker inject PGUSER/PGPASSWORD/PGHOST/PGPORT/PGDATABASE
into the child process. audit-core only accepted AUDIT_CORE_DATABASE_URL, so
consuming a brokered lease would have meant assembling a DSN by hand from the
injected variables - putting the credential back into audit-core's own
configuration, which is what the lane exists to avoid.

An empty conninfo lets libpq read those variables directly, so a brokered lease
now needs no DSN at all. AUDIT_CORE_DATABASE_URL still works for local and test
use. Missing both is a clear startup error naming each option.

Tests 80 -> 82.

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

audit_core/postgres_backend.py

@@ -140,9 +140,17 @@ class PostgresAuditBackend:
         statement_timeout_ms: int = 30_000,
         migrate: bool = True,
     ) -> None:
-        self.dsn = dsn or os.environ.get("AUDIT_CORE_DATABASE_URL") or ""
-        if not self.dsn:
-            raise ValueError("a DSN is required (AUDIT_CORE_DATABASE_URL)")
+        # An empty conninfo is valid: libpq then reads PGHOST/PGUSER/PGPASSWORD/
+        # PGPORT/PGDATABASE from the environment. That is exactly the shape the
+        # railiance-platform credential broker injects into a child process, so
+        # a brokered lease needs no DSN assembled by hand — and no credential
+        # ever passes through audit-core's own configuration.
+        self.dsn = dsn if dsn is not None else os.environ.get("AUDIT_CORE_DATABASE_URL", "")
+        if not self.dsn and not _libpq_env_present():
+            raise ValueError(
+                "no connection information: set AUDIT_CORE_DATABASE_URL, or supply "
+                "PGHOST/PGUSER/PGDATABASE (as the credential broker does)"
+            )
         if not schema.isidentifier():
             raise ValueError(f"unsafe schema name: {schema!r}")
         self.schema = schema
@@ -393,6 +401,11 @@ class PostgresAuditBackend:
             raise BackendUnavailableError(str(exc)) from exc
 
 
+def _libpq_env_present() -> bool:
+    """Whether libpq has enough in the environment to connect on its own."""
+    return bool(os.environ.get("PGHOST") and os.environ.get("PGUSER"))
+
+
 def _timestamp(value: str | None) -> datetime | None:
     if not value:
         return None