replace tokio::broadcast event bus with SQLite-backed event queue (ADR-0003)

- DomainEvent: add Serialize/Deserialize
- EventConsumer port: poll_next/ack/nack replacing recv()
- EventEnvelope: wraps event with id/retry_count/created_at
- SqliteEventPublisher/SqliteEventConsumer: INSERT/poll/DLQ
- migration: event_queue + dead_letter_queue tables
- InMemoryEventBus: combined publisher+consumer test double
- NoopEventConsumer: test stub
- webhook_consumer: polls EventConsumer instead of broadcast::Receiver
- presentation+mcp wiring: create from SqlitePool

migrations_sqlite/20260712000001_add_event_queue.sql

@@ -0,0 +1,25 @@
+CREATE TABLE IF NOT EXISTS event_queue (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    event_type TEXT NOT NULL,
+    payload TEXT NOT NULL,
+    status TEXT NOT NULL DEFAULT 'pending',
+    retry_count INTEGER NOT NULL DEFAULT 0,
+    max_retries INTEGER NOT NULL DEFAULT 3,
+    created_at TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at TEXT NOT NULL DEFAULT (datetime('now')),
+    error_message TEXT
+);
+
+CREATE TABLE IF NOT EXISTS dead_letter_queue (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    original_event_id INTEGER NOT NULL,
+    event_type TEXT NOT NULL,
+    payload TEXT NOT NULL,
+    error_message TEXT NOT NULL,
+    retry_count INTEGER NOT NULL,
+    created_at TEXT NOT NULL DEFAULT (datetime('now')),
+    original_created_at TEXT NOT NULL
+);
+
+CREATE INDEX idx_event_queue_status ON event_queue(status);
+CREATE INDEX idx_event_queue_created_at ON event_queue(created_at);