P2-05: forget command with retention policy

End-to-end forget plumbing — operator can create a forget schedule
with keep-* values, agent runs restic forget --keep-* … on the
schedule's cron (or via per-row Run-now), snapshot list shrinks,
UI updates.

* api.CommandRunPayload gains retention_policy json.RawMessage so
  the agent doesn't need a typed copy of the server-side struct.
* restic.ForgetPolicy mirrors restic's --keep-* flags. Empty()
  reports zero dimensions; restic wrapper RunForget refuses to
  run an empty policy (would delete every snapshot). Does NOT
  pass --prune — pruning lives behind a separate admin-only
  credential (P2-06); forget just rewrites the snapshot index.
* runner.RunForget mirrors RunBackup's envelope shape so the
  live log viewer works without special-casing. On success
  triggers reportSnapshots (forget shrinks the index, the host's
  snapshot count almost certainly changed).
* cmd/agent dispatcher handles MsgCommandRun with kind=forget,
  decodes RetentionPolicy from the wire, builds restic.ForgetPolicy.
* Server dispatchScheduleNow marshals the schedule's
  RetentionPolicy into the wire payload for kind=forget jobs.
  Refuses to dispatch a forget schedule with empty retention.
* validateSchedule rejects kind=forget without at least one keep-*
  dimension (new error code: missing_retention).
* UI schedule edit form gains a Kind dropdown (backup or forget;
  immutable on edit). Paths block toggles by kind via inline
  data-kind attributes. Form help-text explains the prune
  separation.

Other kinds (prune, check, unlock) deferred to P2-06..08; the
Kind dropdown only offers backup and forget today.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/pages/schedule_edit.html

@@ -37,7 +37,31 @@
 
     <div class="col-span-7 panel rounded-[7px] px-8 py-7">
 
-      <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4">When</h3>
+      <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4">Kind</h3>
+      <div class="mb-7">
+        {{if $page.IsNew}}
+          <label class="field-label" for="se-kind">What does this schedule do?</label>
+          <select id="se-kind" name="kind" class="field mono"
+                  onchange="document.querySelectorAll('[data-kind]').forEach(el => { el.style.display = el.dataset.kind === this.value ? '' : 'none'; });">
+            <option value="backup" {{if eq $page.Kind "backup"}}selected{{end}}>backup — snapshot the configured paths</option>
+            <option value="forget" {{if eq $page.Kind "forget"}}selected{{end}}>forget — apply retention policy (rewrite the snapshot index)</option>
+          </select>
+          <div class="field-help">
+            <span class="mono text-ink-mid">backup</span> reads files and writes a snapshot.
+            <span class="mono text-ink-mid">forget</span> trims the index by your <strong>Keep-*</strong> rules without deleting data —
+            an admin-only <span class="mono text-ink-mid">prune</span> job (P2-06) reclaims the disk space later.
+            Other kinds (<span class="mono text-ink-mid">prune</span>, <span class="mono text-ink-mid">check</span>, <span class="mono text-ink-mid">unlock</span>) land in P2-06..08.
+          </div>
+        {{else}}
+          <input type="hidden" name="kind" value="{{$page.Kind}}">
+          <div class="text-[13px] text-ink-mid">
+            Kind: <span class="mono text-ink">{{$page.Kind}}</span>
+            <span class="text-ink-fade">— immutable on edit; delete and recreate to switch kind.</span>
+          </div>
+        {{end}}
+      </div>
+
+      <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4 pt-6 border-t border-line-soft">When</h3>
 
       <div class="mb-5">
         <label class="flex items-center gap-2.5 cursor-pointer text-[13px]">
@@ -65,20 +89,22 @@
         </div>
       </div>
 
-      <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4 pt-6 border-t border-line-soft">Paths</h3>
-      <div class="mb-5">
-        <label class="field-label" for="se-paths">Backup paths <span class="text-ink-fade font-normal">· one per line</span></label>
-        <textarea id="se-paths" name="paths" rows="4" class="field mono"
-                  style="resize: vertical;"
-                  placeholder="/etc&#10;/home&#10;/var/lib/postgresql">{{$page.PathsRaw}}</textarea>
-        <div class="field-help">What <span class="mono text-ink-mid">restic backup</span> walks. The agent runs as root with <span class="mono text-ink-mid">CAP_DAC_READ_SEARCH</span>, so any readable path is fair game.</div>
-      </div>
-      <div class="mb-7">
-        <label class="field-label" for="se-excludes">Excludes <span class="text-ink-fade font-normal">· optional, one per line</span></label>
-        <textarea id="se-excludes" name="excludes" rows="3" class="field mono"
-                  style="resize: vertical;"
-                  placeholder="*.tmp&#10;node_modules&#10;.cache">{{$page.ExcludesRaw}}</textarea>
-        <div class="field-help">Passed straight through as <span class="mono text-ink-mid">--exclude</span> args.</div>
+      <div data-kind="backup" {{if ne $page.Kind "backup"}}style="display: none;"{{end}}>
+        <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4 pt-6 border-t border-line-soft">Paths</h3>
+        <div class="mb-5">
+          <label class="field-label" for="se-paths">Backup paths <span class="text-ink-fade font-normal">· one per line</span></label>
+          <textarea id="se-paths" name="paths" rows="4" class="field mono"
+                    style="resize: vertical;"
+                    placeholder="/etc&#10;/home&#10;/var/lib/postgresql">{{$page.PathsRaw}}</textarea>
+          <div class="field-help">What <span class="mono text-ink-mid">restic backup</span> walks. The agent runs as root with <span class="mono text-ink-mid">CAP_DAC_READ_SEARCH</span>, so any readable path is fair game.</div>
+        </div>
+        <div class="mb-7">
+          <label class="field-label" for="se-excludes">Excludes <span class="text-ink-fade font-normal">· optional, one per line</span></label>
+          <textarea id="se-excludes" name="excludes" rows="3" class="field mono"
+                    style="resize: vertical;"
+                    placeholder="*.tmp&#10;node_modules&#10;.cache">{{$page.ExcludesRaw}}</textarea>
+          <div class="field-help">Passed straight through as <span class="mono text-ink-mid">--exclude</span> args.</div>
+        </div>
       </div>
 
       <h3 class="text-[13px] font-semibold uppercase tracking-[0.08em] text-ink-mute mb-4 pt-6 border-t border-line-soft">Tags <span class="text-ink-fade font-normal">· optional</span></h3>