feat(pairing): solo sealed answers that unlock on pairing

Unpaired users could only reach a waiting-room dead end and could not answer
questions until a partner joined. Responses.couple_id is now nullable so a
solo user answers immediately and banks "sealed answers" (couple_id IS NULL).
Pairing claims both members' sealed answers and computes matches for every
question both have now answered. Invite copy and the unpaired push nudge
escalate with the sealed answer count.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/question-selection.md

@@ -139,11 +139,16 @@ const sorted = filtered.sort((a, b) => {
 
 | Scenario | Behavior |
 |----------|----------|
-| No packs enabled | Shows "No packs enabled" prompt |
+| No packs enabled | Shows "No packs enabled" prompt (paired couples only; a couple with no configured packs has not chosen yet) |
 | All questions answered | Shows "All caught up!" with refresh button |
-| No partner paired | Shows pairing prompt |
+| No partner paired | Answers straight into the swipe flow instead of a blocking prompt. Every answer is sealed (`couple_id IS NULL`, no match). A persistent banner reports the sealed count with an invite CTA. See [Solo answering and sealed answers](couple-pairing.md#solo-answering-and-sealed-answers). |
 | Pack disabled mid-session | Client filters out questions from disabled packs |
 
+An unpaired user has no `couple_packs` row to configure (it is keyed to
+`couple_id`), so `recommended()`'s "unconfigured couple" fallback (every
+public pack) is what they always see; the pack-selection screen only applies
+once paired.
+
 ## Configuration
 
 | Setting | Value | Location |