fix(whatsapp): add outbound retry backoff for failed sends

Track send attempts and schedule retry windows for pending WhatsApp outbound messages, then mark as failed after max retries to avoid silent drops without infinite resend loops.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_channel_outbound_retry.py

@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+from svc.persistence.sqlite_store import SqliteStore
+
+
+def _row(store: SqliteStore, msg_id: str) -> dict:
+    with store._connect() as conn:  # noqa: SLF001 - test introspection
+        got = conn.execute(
+            """
+            SELECT id, status, error, send_attempts, next_attempt_at, last_attempt_at
+            FROM channel_outbound_message
+            WHERE id = ?
+            """,
+            (msg_id,),
+        ).fetchone()
+    assert got is not None
+    return dict(got)
+
+
+def test_channel_outbound_retry_transitions(tmp_path) -> None:
+    store = SqliteStore(str(tmp_path / "ops.sqlite"))
+    msg_id = store.enqueue_channel_outbound_message(
+        channel="whatsapp",
+        account_id="wa-default",
+        chat_id="8615601877957@s.whatsapp.net",
+        text="hello",
+    )
+
+    assert store.ack_channel_outbound_message(message_id=msg_id, ok=False, error="net down") is True
+    first = _row(store, msg_id)
+    assert first["status"] == "pending"
+    assert int(first["send_attempts"] or 0) == 1
+    assert str(first["next_attempt_at"] or "").strip()
+
+    assert store.ack_channel_outbound_message(message_id=msg_id, ok=False, error="still down") is True
+    assert store.ack_channel_outbound_message(message_id=msg_id, ok=False, error="final fail") is True
+    final = _row(store, msg_id)
+    assert final["status"] == "failed"
+    assert int(final["send_attempts"] or 0) == 3
+    assert not str(final["next_attempt_at"] or "").strip()
+
+
+def test_channel_outbound_retry_success_clears_schedule(tmp_path) -> None:
+    store = SqliteStore(str(tmp_path / "ops.sqlite"))
+    msg_id = store.enqueue_channel_outbound_message(
+        channel="whatsapp",
+        account_id="wa-default",
+        chat_id="8615601877957@s.whatsapp.net",
+        text="hello",
+    )
+    assert store.ack_channel_outbound_message(message_id=msg_id, ok=False, error="temp") is True
+    assert store.ack_channel_outbound_message(message_id=msg_id, ok=True, stanza_id="stanza-1") is True
+    row = _row(store, msg_id)
+    assert row["status"] == "sent"
+    assert int(row["send_attempts"] or 0) == 2
+    assert not str(row["next_attempt_at"] or "").strip()