feature/ display timestamps on feed and details page

[irenecancode]

Summary

• Display when each sighting was posted, on both the Wildlife Feed grid cards and the sighting detail page. Previously the calendar icon next to the username rendered empty; there was no timestamp data wired up to it.
• Format: absolute date/time as the primary text (e.g. "Jul 9, 2026, 4:09 PM"), with a relative "X ago" hint shown alongside it. Server-rendered cards use Django's humanize naturaltime filter; AJAX-loaded ("Load More") cards use a matching JS helper so both look identical.

Dependency

This PR only adds the display logic. It depends on a companion backend change (not yet submitted) that adds a created field to the feed/post API responses. Until that backend change ships, post.created will be empty and the calendar icon will still render blank -- same as today, just no regression. Nothing will look different in production from merging this alone.

Test plan

This PR only adds display logic; it has no visible effect until the companion backend PR (adds created to the API response) also merges. Testing splits into two parts:

Verifiable now, against current main (no backend change yet):

• Load /feed/ and confirm no errors/crashes -- cards render exactly as before (calendar icon still blank, since post.created is still absent)
• Load a sighting detail page and confirm the same — no regression from today's behavior

Requires a backend change (not yet started -- pending engineer sign-off), to add a created field to the feed/post API responses:

• Already manually verified in dev: with a local, uncommitted backend patch adding the created field, feed cards correctly showed absolute + relative time (e.g. "Jul 9, 2026, 4:09 PM (5 hours ago)")
• Once that backend change is approved and merged, re-verify on staging/main: feed cards and detail page both show the timestamp correctly
• Confirm "Load More" (AJAX-loaded cards) renders the same format as the initial server-rendered cards

Timestamp display shown above ("Jul 9, 2026, 4:09 PM (5 hours ago)") was captured with a local, uncommitted backend patch adding the created field included here as a preview of the intended result once the companion backend PR ships. Not reproducible against main yet.

The broken image icon is unrelated to this PR — a local-dev-only issue caused by missing GCS credentials on this machine, not something this PR introduces or fixes.