feat: track watchlist completions

internal/database/migrations/026_add_watchlist_completion_dates.sql

@@ -0,0 +1,32 @@
+-- +goose Up
+ALTER TABLE watch_list_entry ADD COLUMN completed_at DATETIME;
+ALTER TABLE watch_list_entry ADD COLUMN completed_at_estimated BOOLEAN NOT NULL DEFAULT 0;
+
+UPDATE watch_list_entry
+SET completed_at = COALESCE(
+        (
+            SELECT MAX(a.occurred_at)
+            FROM audit_log a
+            WHERE a.user_id = watch_list_entry.user_id
+              AND a.action = 'watch_completed'
+              AND a.resource_type = 'anime'
+              AND a.resource_id = CAST(watch_list_entry.anime_id AS TEXT)
+        ),
+        watch_list_entry.updated_at
+    ),
+    completed_at_estimated = CASE
+        WHEN EXISTS (
+            SELECT 1
+            FROM audit_log a
+            WHERE a.user_id = watch_list_entry.user_id
+              AND a.action = 'watch_completed'
+              AND a.resource_type = 'anime'
+              AND a.resource_id = CAST(watch_list_entry.anime_id AS TEXT)
+        ) THEN 0
+        ELSE 1
+    END
+WHERE status = 'completed';
+
+-- +goose Down
+ALTER TABLE watch_list_entry DROP COLUMN completed_at_estimated;
+ALTER TABLE watch_list_entry DROP COLUMN completed_at;