perf(feed): published_at NOT NULL + range-anchored keyset for deep pages (E2 S2, PERF-15)

A nullable published_at forced the feed keyset into a non-sargable
`... OR published_at IS NULL` disjunction: with the 0067 index the planner still
applied it as a FILTER and scanned from the top of the index, so a deep `newest`
page discarded every earlier row (O(depth)).

- Migration 0068: backfill any NULLs (0 in prod) -> SET NOT NULL + DEFAULT now().
- Model: Video.published_at is now non-Optional, nullable=False, server_default now().
- feedquery: the `pub` key is nullable=False, and keyset_predicate is rebuilt
  recursively so a non-nullable leading key gets a non-strict range bound
  (`published_at <= v`) as an AND conjunct — an index range Cond instead of a
  filter. Nullable keys keep the NULL-aware OR form (a `<= v` bound would drop
  their NULL group). Equivalence proven by test_feed_keyset (all 13 sorts,
  cursor-walk == unpaged).
- Ingest paths (stub_from_playlist_item, apply_video_details, playlists sync)
  fall back to now() so they never write an explicit NULL.

Measured on the prod-sized localdev reseed at a ~50k-deep cursor (0067 index +
prod planner): OLD `Rows Removed by Filter: 50001`, 25,687 buffers / 9.16 ms ->
NEW `Index Cond: published_at <= v`, 1 row filtered, 47 buffers / 0.037 ms. SET
NOT NULL on 257k rows: 0.32s. Gate green (334 pytest).

backend/tests/test_feed_keyset.py

@@ -41,14 +41,16 @@ def _seed(db):
         Subscription(user_id=user.id, channel_id="UCa", priority=5, hidden=False),
         Subscription(user_id=user.id, channel_id="UCb", priority=0, hidden=False),
     ])
-    # 7 normal/organic videos. Deliberate duplicates (published_at day 1 x2 and day 3 x2, view_count
-    # 100 x2, duration 60 x2 / 30 x2) and NULLs spread across the sort columns.
+    # 7 normal/organic videos. Deliberate duplicates (published_at day 1 x2, day 2 x2 and day 3 x2,
+    # view_count 100 x2, duration 60 x2 / 30 x2) and NULLs spread across the still-nullable sort columns
+    # (view_count / duration_seconds / title / subscriber_count) to exercise nulls_last. published_at is
+    # NOT NULL (migration 0068), so no NULL there — the duplicates keep the id tiebreaker load-bearing.
     rows = [
         # id       ch     published    views  dur   title
         ("vid00", "UCa", _dt(1),      100,   60,   "alpha"),
         ("vid01", "UCa", _dt(1),      100,   60,   "beta"),
         ("vid02", "UCa", _dt(2),      None,  None, "gamma"),
-        ("vid03", "UCb", None,        50,    120,  None),
+        ("vid03", "UCb", _dt(2),      50,    120,  None),
         ("vid04", "UCb", _dt(3),      200,   30,   "delta"),
         ("vid05", "UCb", _dt(3),      None,  30,   "epsilon"),
         ("vid06", "UCa", _dt(28),     None,  None, "zeta"),