feat: spirit engine — seance WS, entity minting/Codex, Piper TTS voices, wire telemetry

- WS /ws/session: modes, summon, anomaly fragments, streaming direct contact,
  passive wire-ghost ambient loop, per-user rate limits, event transcript
- entities: anomaly-signature fingerprinting, LLM minting + procedural
  fallback, Codex matching with contact counts and sightings
- tts: 8 local Piper voices (EN/ES), per-entity voice profiles, numpy
  effects chain (pitch/rate/bitcrush/echo/static)
- llm: streaming client, submit_stream in bounded queue, SpiritService
  with offline fallbacks for every channel
- routes: public /api/codex, /api/codex/{id}, /api/stats; /audio static mount
- models: Entity, EntitySighting, Event, ContactSession(entity_id, language)

backend/app/models/contact_session.py

@@ -0,0 +1,23 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import DateTime, ForeignKey, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class ContactSession(Base):
+    __tablename__ = "contact_sessions"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"))
+    entity_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("entities.id"), nullable=True
+    )
+    mode: Mapped[str] = mapped_column(String(32), default="unknown")
+    language: Mapped[str] = mapped_column(String(8), default="en")
+    started_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    ended_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)