Add official US and European collection and calendar storage

Collect official RSS and Atom summaries, preserve historical captures, and poll macro releases with bounded cadence and Retry-After handling. Separate news collectors from the economic calendar worker.

Track statistical periods, publication stages, first observations, revisions, and current official series. Include explicit migration files and isolated archive, parsing, and import tests; no migration or import is executed by this commit.

Keep dependency declarations and lockfile together for PDF archives and the upcoming captcha integration. Verify the staged snapshot with 90 focused tests.

backend/tests/fixtures/calendar/fed-month.html

@@ -0,0 +1,14 @@
+<html><body><h3>October 2026</h3><div class="cal-nojs__rowTitle"><h4>FOMC Meetings</h4></div><div class="row">
+                                <div class="col-xs-2">
+                                    <p>2:00 p.m.</p>
+                                </div>
+                                <div class="col-xs-7">
+                                    <p>FOMC Meeting</p>
+                                    <p></p><p>Two-day meeting, October 27 - 28</p>
+
+<p>Press Conference</p>
+                                </div>
+                                <div class="col-xs-3">
+                                    <p>28</p>
+                                </div>
+                            </div></body></html>