Fix Feature 12 review findings F2–F8 on Regular grounding and onboarding

Grounding (F2–F5):
- Each ground_dinner job carries the dinner's new grounding_revision
  (migration 0010). Renaming a pending Regular raises the revision, and a
  job only grounds and writes a dinner still at its revision, so a queued
  original job no longer grounds the new name or decides the outcome.
- Background grounding keeps a method, minutes or difficulty a member saved
  while the Regular was pending, and fills only those left empty.
- Grounding by hand clears the pasted recipe_url, and link deduplication
  uses the source_url of the attached recipe, else the pending link.
- Archived dinners are not grounded: the job checks before any search,
  fetch or model call, and its final update requires an unarchived row.

Onboarding (F6, F7):
- Step 3 shows "Add Regulars" back to step 2 when the server refuses to
  finish with fewer than three Regulars.
- Common picks are hidden by a copy of harness Normalize/Similar, checked
  against the same shared cases as the Go code.

Docs (F8): ARCHITECTURE.md covers the three-Regular minimum, step-3
recovery, the grounding revision and the dedupe rules.

F1 was fixed in 1304487; its test now also asserts the old link is never
grounded and recipe_url stays cleared.

Maestro task 5037ff86-181d-448e-927b-c2fd2258a25d.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/db_test.go

@@ -268,7 +268,7 @@ func TestDinners(t *testing.T) {
 	}
 	var unarchivedIDs []string
 	for _, d := range unarchived {
-		unarchivedIDs = append(unarchivedIDs, d.ID)
+		unarchivedIDs = append(unarchivedIDs, d.Dinner.ID)
 	}
 	assertIDs(t, "unarchived", unarchivedIDs, other.ID, third.ID)
 	_, err = q.ArchiveDinner(ctx, db.ArchiveDinnerParams{ID: "missing", UpdatedAt: archivedAt})