feat: Phase 5 — events system complete

Adds a data-driven event system (#47 schema, #48 activation, #49 TOTW):

- EventDefinition JSON schema (event_type, effects, date range, is_manual)
- data/events/totw_week1.json — TOTW event active by date range (2024–2030),
  injects 5 TOTW cards into the market as [EVENT] listings
- data/events/seasonal_events.json — Spring Festival and Icon Weekend,
  manual-activation-only example events
- migrations/0004_events.sql — events override table (NULL/1/0 per event)
- services/event — load_event_definitions, compute_is_active (date range +
  manual override), get_active_events, get_all_with_status, activate/deactivate
- routes/events — GET /events, GET /events/:id, POST .../activate, .../deactivate
- services/market::refresh_npc_listings now accepts event_defs, injects bonus
  market cards from active events at 2× premium price
- 5 new integration tests (events list, single, activate/deactivate cycle,
  404 on unknown, market injection verification); 19/19 passing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

migrations/0004_events.sql

@@ -0,0 +1,8 @@
+-- Manual override table for event activation/deactivation.
+-- is_active_override: NULL = use date range, 1 = force active, 0 = force inactive
+CREATE TABLE IF NOT EXISTS events (
+    id TEXT PRIMARY KEY,
+    is_active_override INTEGER,
+    activated_at TEXT,
+    deactivated_at TEXT
+);