M1: backend core + proposal engine

- FastAPI skeleton, SQLAlchemy models (§13), Alembic initial migration
- SchedulingProvider interface with google_calendar (free/busy read-only),
  partner_api (Appendix B client) and mock implementations
- Proposal engine: create → provider-routed delivery → owner actions
  (resolve/confirm+SMS/reject) → expiry + reminders (§9)
- Signed single-use action links, .ics METHOD:REQUEST attachment
- Partner outcome webhook with HMAC verification + polling fallback
- SmsProvider (console) with Bosnian templates (§5.5), EmailProvider (console/SMTP)
- Fake partner API server in tests/ — Appendix B reference implementation
- 43 tests: slot math, proposal lifecycle, action links, partner contract

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

gogo/sms/base.py

@@ -0,0 +1,72 @@
+"""SmsProvider interface. Console impl for dev/tests; GSM gateway impl in gateway.py (M5)."""
+
+from __future__ import annotations
+
+import logging
+import uuid
+from typing import Protocol
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from gogo.config import get_settings
+from gogo.models import SmsLog
+
+log = logging.getLogger("gogo.sms")
+
+
+class SmsProvider(Protocol):
+    async def send(self, tenant_id: uuid.UUID, to_msisdn: str, body: str) -> None:
+        """Send one SMS from the tenant's assigned SIM. Raises on failure."""
+        ...
+
+
+class ConsoleSmsProvider:
+    """Logs SMS instead of sending; keeps them in memory for tests."""
+
+    sent: list[tuple[str, str]]
+
+    def __init__(self) -> None:
+        self.sent = []
+
+    async def send(self, tenant_id: uuid.UUID, to_msisdn: str, body: str) -> None:
+        self.sent.append((to_msisdn, body))
+        log.info("SMS tenant=%s to=%s: %s", tenant_id, to_msisdn, body)
+
+
+_provider: SmsProvider | None = None
+
+
+def get_sms_provider() -> SmsProvider:
+    global _provider
+    if _provider is None:
+        if get_settings().sms_provider == "gsm_gateway":
+            from gogo.sms.gateway import GsmGatewaySmsProvider
+
+            _provider = GsmGatewaySmsProvider()
+        else:
+            _provider = ConsoleSmsProvider()
+    return _provider
+
+
+def set_sms_provider(p: SmsProvider | None) -> None:
+    """Test hook."""
+    global _provider
+    _provider = p
+
+
+async def send_sms(
+    session: AsyncSession, tenant_id: uuid.UUID, to_msisdn: str, body: str, kind: str
+) -> bool:
+    """Send + log. Returns success. Never raises (SMS failure must not break the flow)."""
+    row = SmsLog(tenant_id=tenant_id, to_msisdn=to_msisdn, body=body, kind=kind)
+    try:
+        await get_sms_provider().send(tenant_id, to_msisdn, body)
+        row.status = "sent"
+        ok = True
+    except Exception as e:  # noqa: BLE001
+        log.exception("SMS send failed tenant=%s to=%s", tenant_id, to_msisdn)
+        row.status = "failed"
+        row.error = str(e)
+        ok = False
+    session.add(row)
+    return ok