feat(P01-04-01,P01-04-02): SQLite schema + async store (D-007)

db/schema.sql + db/migrations/0001_init.sql — four tables: learner (single
hardcoded 'learner-1'/'Alex' row, D-007 no auth), sessions (id, learner_id,
scenario_id, started_at, ended_at, branch_path_json, outcome,
cost_estimated_cents, debrief_text, cost_breakdown_json), turns (id,
session_id, seq, role, asr_text, tts_text, latency_ms), progress (learner_id,
scenario_id, attempts, last_outcome). db/migrate.py applies migrations
idempotently via a _migrations tracking table.

db/store.py — PraxisStore async access layer (aiosqlite): start_session,
log_turn, end_session (branch_path + outcome + cost + debrief),
update_progress (increment attempts + last_outcome), get_session, get_turns,
get_learner. Type-annotated SessionRow/TurnRow dataclasses. 6 tests pass
(migration creates all tables, hardcoded learner exists, idempotent
migrations, start→log→end→query full session, update_progress, get_learner).

---ci---
phase: 1
milestone: v0.1
plan: 04
task: 04-01,04-02
status: execute
persona: data-engineer
requirements:
  covered: [REQ-STATE-01]
---/ci---

db/__init__.py

@@ -0,0 +1,17 @@
+"""Praxis SQLite store package — async access layer (D-007)."""
+
+from db.store import (
+    PraxisStore,
+    SessionRow,
+    TurnRow,
+    HARDCODED_LEARNER_ID,
+)
+from db.migrate import apply_migrations
+
+__all__ = [
+    "PraxisStore",
+    "SessionRow",
+    "TurnRow",
+    "HARDCODED_LEARNER_ID",
+    "apply_migrations",
+]