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/app/config.py

@@ -1,5 +1,5 @@
 from pydantic import model_validator
-from pydantic_settings import BaseSettings
+from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
 class Settings(BaseSettings):
@@ -39,8 +39,7 @@ class Settings(BaseSettings):
             raise ValueError("ADMIN_PASSWORD must be set and must not be 'admin'")
         return self
 
-    class Config:
-        env_file = ".env"
+    model_config = SettingsConfigDict(env_file=".env")
 
 
 settings = Settings()