chore: dead-code + duplicate sweep; fix moon-flaky re-contact test

Dead code: removed an unused `settings` import (main.py), an unused
`RITUAL_HOLD_MS` import (RitualPanel), and an unused `beforeEach`
(SigilDesigner test). The `_refs` keep-alive in sdr.ts is deliberate
(holds hardware-pass constants) and stays; the orphaned .evp-scope /
.radio-waterfall CSS was already removed in an earlier lint pass.

Duplicate: coldSpot.ts and baseline.ts each carried the same time-aware
EMA alpha formula. Extracted it as baseline.emaAlpha(dtMs, tauMs) and
pointed both at it. coldSpot's pure-function core is deliberately NOT
merged into the stateful ThresholdBaseline class — different contract
(immutable-state-threaded vs internal-threshold), and forcing them
together would be an overhaul that risks the tested cold-spot logic.

Determinism fix: test_familiar_presence_answers_again_on_a_known_channel
pinned RETURN_CHANCE=1.0 but not the sky. Since the astronomy wiring made
the real return chance RETURN_CHANCE*(1 - veil_thinness*PULL), and
veil_thinness reads the *actual current moon phase*, a full-moon test run
dragged the effective chance to ~0.55 and the test failed ~45% of the
time. Now also pins VEIL_THINNESS_PULL=0 to isolate re-contact from the
veil influence (which has its own tests). Verified 12/12 consecutive
passes; it was ~7/12 before.

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

backend/app/main.py

@@ -9,7 +9,6 @@ from fastapi.staticfiles import StaticFiles
 from sqlalchemy import text
 
 import app.models  # noqa: F401 — registers models on Base.metadata before create_all
-from app.config import settings
 from app.db import Base, async_session_maker, engine
 from app.routes.auth import router as auth_router
 from app.routes.codex import router as codex_router