Fix domain reports and sources API 500 errors

- Use begin_timestamp/end_timestamp (Unix ints) instead of begin_date/end_date
  (ISO strings) when building ReportEntry, fixing Pydantic int_parsing errors
- Extract policy string from dict (policy["p"]) when the stored value is a dict,
  fixing Pydantic string_type validation error
- Rename _days -> days in ReportStore.get_domain_sources() so the endpoint's
  `days=days` keyword call no longer raises TypeError

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/15ac3521-e81f-42fa-81a2-f6804a423e1d

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/api/api_v1/endpoints/domains.py

@@ -371,15 +371,18 @@ async def get_domain_reports(
     # Generate report entries
     report_entries = []
     for report in reports:
+        policy_val = report.get("policy", "none")
+        if isinstance(policy_val, dict):
+            policy_val = policy_val.get("p", "none")
         report_entries.append(
             ReportEntry(
                 id=report.get("report_id", "unknown"),
                 org_name=report.get("org_name", "Unknown Organization"),
-                begin_date=report.get("begin_date", 0),
-                end_date=report.get("end_date", 0),
+                begin_date=report.get("begin_timestamp", 0),
+                end_date=report.get("end_timestamp", 0),
                 total_emails=report.get("total_count", 0),
                 pass_rate=report.get("pass_rate", 0.0),
-                policy=report.get("policy", "none"),
+                policy=policy_val,
             )
         )