Implement Phase 0b base project against the frozen test contract

All seven services (gateway, ledger, fraud, sanctions, insights, notifier,
audit), db/schema.sql plus optional_indexes.sql, scripts/seed.py and
smoke.py, the gateway-served frontend, and docker-compose.yml. Plain-text
stdlib logging and a weak manual X-Request-Id only -- no observability yet,
by design, per test_no_observability.py. Implements all 13 named chaos-flag
pathologies from docs/PATHOLOGIES.md behind Redis-backed runtime flags.

make smoke and make test-contract pass; make test-baseline and
make test-pathology pass except for a handful of load-sensitive tests
documented in tests/README.md rather than papered over.

services/notifier/consumer.py

@@ -0,0 +1,78 @@
+import asyncio
+import logging
+import random
+import uuid
+
+import sqlalchemy as sa
+
+from common.db import Session
+from common.redis_client import redis_client
+from common.settings import settings
+
+from . import chaos
+
+logger = logging.getLogger("notifier")
+
+GROUP = "notifier"
+CONSUMER = "notifier-1"
+
+
+async def ensure_group():
+    try:
+        await redis_client.xgroup_create(settings.stream_name, GROUP, id="$", mkstream=True)
+    except Exception as exc:
+        if "BUSYGROUP" not in str(exc):
+            raise
+
+
+async def _handle(fields: dict):
+    if await chaos.slow_consumer_enabled():
+        await asyncio.sleep(chaos.SLEEP_SECONDS)
+
+    async with Session() as session:
+        await session.execute(
+            sa.text(
+                "INSERT INTO notifications (id, transaction_id, channel, status, attempts, sent_at) "
+                "VALUES (:id, :tx_id, :channel, 'sent', 1, now())"
+            ),
+            {
+                "id": str(uuid.uuid4()),
+                "tx_id": fields.get("transaction_id"),
+                "channel": random.choice(["email", "push", "sms"]),
+            },
+        )
+        await session.commit()
+
+
+async def run():
+    """Background consumer loop for the `notifier` consumer group -- a second,
+    independent group (`audit`) reads the same stream in services/audit/worker.py.
+    Redis Streams carry no automatic context propagation, deliberately."""
+    await ensure_group()
+    logger.info("notifier consumer started")
+    while True:
+        try:
+            resp = await redis_client.xreadgroup(
+                groupname=GROUP,
+                consumername=CONSUMER,
+                streams={settings.stream_name: ">"},
+                count=10,
+                block=2000,
+            )
+        except asyncio.CancelledError:
+            raise
+        except Exception:
+            logger.exception("notifier consumer read failed")
+            await asyncio.sleep(1)
+            continue
+
+        if not resp:
+            continue
+
+        for _, messages in resp:
+            for msg_id, fields in messages:
+                try:
+                    await _handle(fields)
+                except Exception:
+                    logger.exception(f"notifier failed to process message {msg_id}")
+                await redis_client.xack(settings.stream_name, GROUP, msg_id)