Add pytest harness and 55-test suite for core backend logic

Covers: billing-cycle characterization (the BE-06 convention, boundary
instants, year wraps, deferred transactions), projection engine
(recurring schedules, no-double-count, cumulative balances, overrides,
fresh start), auth (JWT roundtrip/expiry, API tokens, constant-time
creds, rate-limit window), fail-fast settings, and the BAC paste
parser. SQLite in-memory fixtures, exchange rates pinned — no network.
Also doubles as the float-behavior baseline for the Phase 2 Decimal
migration (plan 1.5). config.py moves to SettingsConfigDict to clear
the pydantic deprecation warning from test output.

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

backend/tests/conftest.py

@@ -0,0 +1,48 @@
+"""Shared fixtures. Env vars are set before any app import so config.Settings()
+fail-fast validation passes with deterministic test values regardless of the
+developer's .env."""
+
+import os
+
+os.environ.setdefault(
+    "SECRET_KEY", "test-secret-key-0123456789abcdef0123456789abcdef"
+)
+os.environ.setdefault("ADMIN_USERNAME", "testadmin")
+os.environ.setdefault("ADMIN_PASSWORD", "test-password")
+os.environ.setdefault("DATABASE_URL", "sqlite://")
+
+import pytest
+from sqlalchemy.pool import StaticPool
+from sqlmodel import Session, SQLModel, create_engine
+
+import app.models.models  # noqa: F401 — register all tables on the metadata
+
+
+@pytest.fixture()
+def engine():
+    eng = create_engine(
+        "sqlite://",
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+    )
+    SQLModel.metadata.create_all(eng)
+    yield eng
+    eng.dispose()
+
+
+@pytest.fixture()
+def session(engine):
+    with Session(engine) as s:
+        yield s
+
+
+@pytest.fixture(autouse=True)
+def fixed_exchange_rates(monkeypatch):
+    """Never hit the network: CRC passthrough, USD pinned at 500."""
+    from app.services import exchange_rate
+
+    monkeypatch.setattr(
+        exchange_rate,
+        "get_crc_multipliers",
+        lambda session: {"CRC": 1.0, "USD": 500.0},
+    )