Segment membership Edge UI

[khvn26]

Scope

• "Refresh" button on the Identities tab (from Segment membership frontend #7412) for envs served by the Edge API. Calls POST /api/v1/environments/{id}/membership/refresh (Segment membership refresh API #7411).
• Staleness emphasis on as_of for edge envs (e.g. relative-time chip styled to draw attention when the timestamp is more than a few minutes old).
• Click-through routing for edge member items: navigate to the edge identity detail page using the composite_key from the member item, via the new lookup added in EdgeIdentityViewSet composite-key lookup #7414.
• Handle the 202 / 429 response shapes from Segment membership refresh API #7411.
• E2E test covering the click-Refresh flow on an edge env.

Acceptance criteria

• Refresh button is shown only for edge envs. Hidden for core envs.
• Clicking Refresh enqueues a backfill (202) and the tab flips into the warming state on response.
• A second click within the cooldown window surfaces the 429 as a non-blocking notice.
• Staleness chip is visible for edge envs and absent for core envs.
• Clicking a member identifier on an edge env navigates to the edge identity detail page and loads the identity successfully.