Backfill Michigan calendar results from official archives

Fetch annual report indexes, skip stored PDFs, and stop the source on rate limits. Match report periods, stages, and publication dates to official schedules while preserving received results and keeping revised monthly series separate.

backend/src/alphapulse/calendar/catalog.py

@@ -87,6 +87,7 @@ UMICH_HOME = "https://www.sca.isr.umich.edu/"
 UMICH_HISTORY = "https://data.sca.isr.umich.edu/technical-docs.php"
 UMICH_INFO = "https://data.sca.isr.umich.edu/survey-info.php"
 UMICH_CSV = "https://www.sca.isr.umich.edu/files/tbmics.csv"
+UMICH_REPORTS = "https://data.sca.isr.umich.edu/reports.php"
 SCHEDULE_URLS = {
     "bls": "https://www.bls.gov/schedule/news_release/bls.ics",
     "bea": "https://www.bea.gov/news/schedule/ics/online-calendar-subscription.ics",