Allow editing an existing metric declaration in place (ticket 022)

Clicking a declared metric badge in Metrics.tsx now loads its regex/params
into the declaration form for editing (in addition to selecting it for the
bucket-values view), and shows those properties inline once selected rather
than only on hover. The backend gains PUT /api/metrics/{name}, backed by a
new engine::update_metric that reuses declare_metric's validation, requires
the metric to already exist, supports renaming, and re-backfills bucket data
from the new regex/params instead of leaving stale buckets from the old one.
Submitting the form while editing calls the update endpoint in place rather
than creating a second declaration; declaring a brand-new metric is unchanged.

Note: tickets/022-edit-existing-metric-declarations.md does not exist on this
worktree's branch (it was added to tickets/ on main after this branch was
created), so there is nothing to delete here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/profile.rs

@@ -541,6 +541,31 @@ impl ProfileStore {
         Ok(())
     }
 
+    /// Same as `record_metric_declaration`, for an in-place edit that may rename the metric: drops any persisted record under `old_name` first (when it differs from the new name) so a rename doesn't leave a stale duplicate declaration behind, then upserts by the new name as usual.
+    pub fn record_metric_declaration_update(
+        &mut self,
+        store_path: &Path,
+        profile_name: &str,
+        old_name: &str,
+        request: &DeclareMetricRequest,
+    ) -> Result<(), ProfileError> {
+        let profile = self
+            .profiles
+            .iter_mut()
+            .find(|p| p.name == profile_name)
+            .ok_or(ProfileError::NotFound)?;
+        if old_name != request.name {
+            profile.metrics.retain(|r| r.name != old_name);
+        }
+        let record: MetricDeclarationRecord = request.into();
+        match profile.metrics.iter_mut().find(|r| r.name == record.name) {
+            Some(existing) => *existing = record,
+            None => profile.metrics.push(record),
+        }
+        self.save(store_path)?;
+        Ok(())
+    }
+
     /// Lists the dashboards saved on `profile_name`; fails with `NotFound` if it doesn't exist.
     pub fn list_dashboards(&self, profile_name: &str) -> Result<&[Dashboard], ProfileError> {
         let profile = self.get(profile_name).ok_or(ProfileError::NotFound)?;