fix: resolve frontend timestamp overwrite, and fix test fixtures

[irenecancode]

Summary

Wires up the post creation timestamp end-to-end on the feed and post detail pages, fixing two silent display bugs uncovered along the way, adding regression coverage for both, and adding a relative-time UX improvement.

Bug fixes

• Feed and post detail pages were displaying the wrong timestamp. _normalize_post() in web_views.py was unconditionally overwriting the API's created field with encounter_datetime (the wildlife sighting time, not the post's creation time) — in both the server-rendered templates and the infinite-scroll JSON endpoint. Removed the overwrite so the real created value reaches the templates.
• The timestamp was actually rendering blank, not just wrong. Django's |date: template filter silently renders "" when given a raw string instead of a datetime object. Since the API client does plain response.json() with no datetime parsing, this had likely been invisible in the feed/detail views for a while. Fixed by parsing created into a proper datetime via parse_datetime() (defensively guarded so it's idempotent regardless of call order).

Test coverage

• FAKE_POST test fixture now includes a created value distinct from encounter_datetime — previously it didn't include created at all, so the existing suite could neither catch the original bug nor verify the fix.
• Added assertions on both affected code paths (server-rendered feed context, and the load-more JSON response) pinning that created — not encounter_datetime — is what's returned.

UX enhancement

• Feed timestamps now show relative time ("now", "an hour ago", "2 days ago", etc.) instead of an absolute date, using Django's built-in humanize.naturaltime filter for the initial page render.
• The infinite-scroll JS path (formatDate()) was rewritten to match naturaltime's output exactly — verified boundary-for-boundary against Django's filter — so timestamps read consistently regardless of how a post entered the feed.

Test plan

• python manage.py test siteapps.socialmedia.test_web_views — all 39 tests passing
• Manually verified relative-time formatting matches between server-rendered and infinite-scroll-loaded posts
• Manual smoke test of feed + post detail pages in a browser

Follow-up (separate PR)

• Align USE_TZ/TIME_ZONE settings between WildeBackyardBackend and WildeBackyardWeb (they currently share a database but disagree on timezone handling)
• Clean up leftover project_restoration scaffold docstrings in WildeBackyardWeb