Add gap-free QueryVault capture windows

[stevewittek]

Summary

• add optional @LookbackMinutes for explicit hour/minute capture windows
• make SQL Agent resume from each database's latest completed endpoint
• retain configured day fallback only for first-time capture
• include the contributor/canonical grants required by the Agent account
• document and statically test the new contracts

Validation

• git diff --check passes
• stored procedure and job scripts parsed successfully against Voyager2 SQL Server
• production recovery runs 114-116 completed for all three databases
• controlled workload/capture validation is running on Voyager2