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.

common/settings.py

@@ -0,0 +1,27 @@
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
+
+    service_name: str = "unknown"
+
+    postgres_dsn: str = "postgresql+asyncpg://ledgerline:ledgerline@postgres:5432/ledgerline"
+    redis_url: str = "redis://redis:6379/0"
+
+    gateway_url: str = "http://gateway:8000"
+    ledger_url: str = "http://ledger:8001"
+    fraud_url: str = "http://fraud:8002"
+    sanctions_url: str = "http://sanctions:8003"
+    insights_url: str = "http://insights:8004"
+    notifier_url: str = "http://notifier:8005"
+
+    downstream_timeout_seconds: float = 3.0
+
+    stream_name: str = "stream:transactions"
+
+    summary_cache_ttl_seconds: int = 900
+    summary_cache_ttl_seconds_cold: int = 60
+
+
+settings = Settings()