Warn template admins that autostop/activity_bump changes don't affect running workspaces #21688

Problem

When a template admin changes default_ttl or activity_bump settings, existing running workspaces retain their pre-existing deadlines. This is intentional behavior (per #2229), but it creates a confusing experience:

1. Admin disables autostop (default_ttl=0) expecting workspaces to run indefinitely
2. Admin also disables activity bump (activity_bump=0)
3. Existing running workspaces still have their old deadlines
4. Since activity_bump=0, user activity can no longer extend those deadlines
5. Workspaces autostop when their old deadline passes, despite the admin believing autostop was disabled

The user appears to be "actively using" the workspace, but activity detection doesn't matter when activity_bump=0.

Why This Happens

Deadlines are stored on the workspace build, not the template. When a build completes, the deadline is calculated from the template settings at that moment:

deadline = build_completed_at + TTL (if TTL > 0)
deadline = zero (if TTL = 0) → means "no autostop"

Changing template settings doesn't retroactively update existing builds. This is documented in the lifecycle executor:

// NOTE: If a workspace build is created with a given TTL and then the user either
//       changes or unsets the TTL, the deadline for the workspace build will not
//       have changed. This behavior is as expected per #2229.

Proposed Solution

UI

• Add a warning banner in the template schedule settings page when changing default_ttl, activity_bump, or autostop requirements
• The warning should explain that running workspaces will keep their existing deadlines until restarted
• Optionally show the count of running workspaces that have active deadlines

Documentation

• Swagger/OpenAPI: Add description annotations to default_ttl_ms and activity_bump_ms fields in UpdateTemplateMeta explaining that changes don't affect running workspaces
• Template scheduling docs: Add a dedicated section on "How deadline changes propagate" with clear examples

CLI

• coder templates edit could print an informational message after successful update when TTL/activity_bump settings are changed

Workaround

Currently, users must restart their workspace after template changes to pick up new autostop settings.

Created on behalf of @bjornrobertsson