feat: event ids, outbox identity migration

crates/adapters/sqlite/migrations/0101_outbox_identity.sql

@@ -0,0 +1,23 @@
+-- Outbox identity: every queued event carries a stable id; handlers record
+-- (event_id, handler) pairs so redeliveries are skipped.
+ALTER TABLE event_queue ADD COLUMN event_id TEXT;
+
+-- Legacy rows get a random v4-shaped uuid. The subquery references the row id
+-- so it is re-evaluated per row rather than cached.
+UPDATE event_queue SET event_id = (
+    SELECT lower(
+        substr(h, 1, 8) || '-' || substr(h, 9, 4) || '-4' || substr(h, 14, 3) || '-'
+        || substr('89ab', (abs(random()) % 4) + 1, 1) || substr(h, 18, 3) || '-' || substr(h, 21, 12)
+    )
+    FROM (SELECT hex(randomblob(16)) AS h, event_queue.id AS row_id)
+);
+
+CREATE UNIQUE INDEX idx_event_queue_event_id
+    ON event_queue (event_id) WHERE event_id IS NOT NULL;
+
+CREATE TABLE processed_events (
+    event_id     TEXT NOT NULL,
+    handler      TEXT NOT NULL,
+    processed_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    PRIMARY KEY (event_id, handler)
+);