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

crates/domain/src/ports/mod.rs

@@ -13,7 +13,7 @@ pub mod user;
 pub use activity::{ActivityLogCommand, ActivityLogQuery};
 pub use auth::{AuthService, TokenService};
 pub use channel::{ChannelCommand, ChannelQuery};
-pub use events::{DomainEvent, EventConsumer, EventHandler, EventPublisher};
+pub use events::{DomainEvent, EventConsumer, EventEnvelope, EventHandler, EventPublisher};
 pub use library::{LibraryCommand, LibraryQuery, LibrarySyncAdapter};
 pub use media::{
     Collection, IMediaProvider, IProviderRegistry, ProviderCapabilities, SeriesSummary,