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.

db/drop_indexes.sql

@@ -0,0 +1,4 @@
+-- Applied only by `make drop-index`. Reverses db/optional_indexes.sql.
+
+DROP INDEX IF EXISTS transactions_account_created_idx;
+DROP INDEX IF EXISTS transactions_category_idx;

db/optional_indexes.sql

@@ -0,0 +1,5 @@
+-- Applied only by `make fix-index` (docs/phase-0-base-project-spec.md §5, §12).
+-- These are the indexes deliberately omitted from db/schema.sql.
+
+CREATE INDEX IF NOT EXISTS transactions_account_created_idx ON transactions (account_id, created_at);
+CREATE INDEX IF NOT EXISTS transactions_category_idx ON transactions (category_id);

db/schema.sql

@@ -0,0 +1,80 @@
+-- Ledgerline schema. Idempotent: safe to run against an existing database.
+-- Index policy is deliberate -- see docs/phase-0-base-project-spec.md §5.
+-- Do NOT add transactions(account_id, created_at) or transactions(category_id)
+-- here; those live in db/optional_indexes.sql, applied only by `make fix-index`.
+
+CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
+
+CREATE TABLE IF NOT EXISTS accounts (
+    id              UUID PRIMARY KEY,
+    holder_name     TEXT NOT NULL,
+    currency        CHAR(3) NOT NULL,
+    balance_minor   BIGINT NOT NULL,
+    is_hot          BOOLEAN NOT NULL DEFAULT FALSE,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE TABLE IF NOT EXISTS merchant_categories (
+    id              SMALLINT PRIMARY KEY,
+    code            TEXT NOT NULL UNIQUE,
+    label           TEXT NOT NULL,
+    risk_weight     NUMERIC(3,2) NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS transactions (
+    id              UUID PRIMARY KEY,
+    account_id      UUID NOT NULL,
+    kind            TEXT NOT NULL,
+    amount_minor    BIGINT NOT NULL,
+    currency        CHAR(3),
+    merchant_name   TEXT,
+    category_id     SMALLINT,
+    status          TEXT NOT NULL,
+    risk_score      NUMERIC(4,3),
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+-- Parallel workers otherwise let a plain sequential scan of this table
+-- compete with an index scan on raw CPU parallelism for higher-selectivity
+-- accounts (e.g. the 5 hot ones, each ~6-8% of all rows) -- masking exactly
+-- the missing-index pathology this table's index policy (README §5) exists
+-- to demonstrate. Scoped to this one table, not a database-wide setting.
+ALTER TABLE transactions SET (parallel_workers = 0);
+
+CREATE TABLE IF NOT EXISTS sanctions_entries (
+    id              SERIAL PRIMARY KEY,
+    normalized_name TEXT NOT NULL,
+    country         CHAR(2),
+    list_source     TEXT,
+    added_at        TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE TABLE IF NOT EXISTS notifications (
+    id              UUID PRIMARY KEY,
+    transaction_id  UUID NOT NULL,
+    channel         TEXT NOT NULL,
+    status          TEXT NOT NULL,
+    attempts        INT NOT NULL DEFAULT 0,
+    sent_at         TIMESTAMPTZ
+);
+
+CREATE TABLE IF NOT EXISTS audit_events (
+    id              BIGSERIAL PRIMARY KEY,
+    transaction_id  UUID NOT NULL,
+    event_type      TEXT NOT NULL,
+    payload         JSONB,
+    recorded_at     TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+-- Not part of the base spec's domain model: a single-row table the seed
+-- script writes its summary manifest into, so both `scripts/smoke.py` and
+-- the test suite can read back what a given seed run actually produced
+-- (row counts, hot/sanctioned/poison account ids, the SEED value used)
+-- without re-deriving it via ad-hoc queries.
+CREATE TABLE IF NOT EXISTS seed_manifest (
+    id      SMALLINT PRIMARY KEY DEFAULT 1,
+    data    JSONB NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS accounts_holder_name_idx ON accounts (holder_name);
+CREATE INDEX IF NOT EXISTS sanctions_entries_normalized_name_idx ON sanctions_entries (normalized_name);