Wire durable delivery outbox: migration, store, and dispatcher config

Companion to the dispatcher-side outbox change: adds the
delivery_attempts table migration, Store.BeginDeliveryAttempt/
CompleteDeliveryAttempt/ReconcileStaleDeliveryAttempts, and wires
ReconcileStaleDeliveryAttempts + Config.Outbox into mavend startup
before the tick loop resumes.

Vikunja #270.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ghELqYhZNLub2TXGMazqA

internal/store/migrations.go

@@ -50,6 +50,19 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 		sent_at   INTEGER NOT NULL
 	);
 	CREATE INDEX IF NOT EXISTS idx_ack_sends_rule ON ack_sends (rule, sent_at DESC);`, // #5 — sev4 telegram repeat-til-ack tracking
+
+	`CREATE TABLE IF NOT EXISTS delivery_attempts (
+		id           INTEGER PRIMARY KEY AUTOINCREMENT,
+		kind         TEXT NOT NULL CHECK (kind IN ('nudge','reminder')),
+		rule         TEXT NOT NULL DEFAULT '',
+		reminder_id  INTEGER NOT NULL DEFAULT 0,
+		channel      TEXT NOT NULL,
+		body_hash    TEXT NOT NULL,
+		status       TEXT NOT NULL DEFAULT 'pending' CHECK (status IN ('pending','sent','failed','unknown')),
+		created_ts   INTEGER NOT NULL,
+		completed_ts INTEGER
+	);
+	CREATE INDEX IF NOT EXISTS idx_delivery_attempts_status ON delivery_attempts (status);`, // #6 — durable delivery outbox
 }
 
 // migrate applies every migration with a number greater than the DB's current