feat: live class sessions, CE workflow, and docs (#683)

Adds live-class session scheduling on lessons (join link, date/time,
timezone) with automatic 24h and 1h reminder emails plus a calendar
invite (.ics), a time-change notification with an updated invite, a
teacher-authored welcome email and session invite on enrollment, and a
"Notify students" quiz nudge.

LMS gets an upcoming live-sessions card and Join buttons, plus a
reusable live-session card component (3 states) in @cio/ui with a
Storybook story. The sidebar content tree now reflects sequential
progression locking like the content list, and the course-completion
confetti regression is fixed.

Org-based emails no longer reference ClassroomIO by name. Docs add
live-class, welcome-email, and course-progression guides, expand the
certificates guide (design, signatures, corrected labels), move
plan-required callouts to the top, flatten how-to-guides to the docs
root with redirects, and add screenshots. Includes an add-docs-image
skill.

.gitignore

@@ -60,4 +60,11 @@ company/
 
 # Claude Code local artifacts
 .claude/skills
-.claude/scheduled_tasks.lock
+.claude/scheduled_tasks.lock
+
+# Local db-agent playground (not committed)
+apps/db-agent
+
+# Generated htpasswd files (hashes are safe to store in secrets/CI, plaintext never committed)
+.htpasswd
+*.htpasswd