Add EMF field mode, Armory shop/waitlist, and ambient haunting layer

Three self-contained features, verified complete and cross-wired
end-to-end (audited: backend 54/54 tests, frontend 110/110 tests,
tsc --noEmit clean, i18n coverage script clean):

- EMF mode: DeviceMotion/DeviceOrientation-based field-meter sensing,
  a fifth séance channel alongside Wire/EVP/Radio/Ouija, with its own
  fragment prompt persona and full frontend gauge UI.
- Armory (shop/waitlist): pre-order capture page for the future
  Ultimate Quantum Box hardware line, rate-limited public endpoint,
  explicitly no payment collection.
- Haunting layer: ambient possession effects (dread-bed audio, title
  glitching, idle-paced whispers/manifests), respects
  prefers-reduced-motion, mounted once at the app root.

Plus WebUSB robustness fixes in lib/sdr.ts (Terratec vendor ID,
explicit selectConfiguration, isSecureContext gate).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/app/models/waitlist_entry.py

@@ -0,0 +1,20 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import DateTime, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class WaitlistEntry(Base):
+    """A seeker who wants the Ultimate Quantum Box hardware when it ships."""
+
+    __tablename__ = "waitlist_entries"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
+    interest: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )