Add reports:backfill_revision task

Fills revision for existing rows by extracting the full commit SHA from
ltsv. Intended to run once on Heroku after the column migration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

lib/tasks/backfill_revision.rake

@@ -0,0 +1,20 @@
+namespace :reports do
+  desc "Backfill reports.revision with full commit SHA extracted from ltsv"
+  task :backfill_revision => :environment do
+    scope = Report.where(revision: nil).where.not(ltsv: nil)
+    total = scope.count
+    puts "#{total} reports to scan"
+    scanned = 0
+    updated = 0
+    scope.find_each do |report|
+      scanned += 1
+      sha = Report.extract_full_sha(report.ltsv)
+      if sha
+        report.update_column(:revision, sha)
+        updated += 1
+      end
+      puts "#{scanned}/#{total} scanned, #{updated} updated" if scanned % 1000 == 0
+    end
+    puts "done: #{scanned} scanned, #{updated} updated"
+  end
+end