docs(adr): ADR-031 inline-expand summary + Q&A in the list (issue #16)

DECISIONS.md

@@ -1219,6 +1219,54 @@ stops scraping; the app is unaffected. No schema change.
 
 ---
 
+## ADR-031 — SPA-like reader: inline-expand summary + Q&A in the list (HTMX, no framework)
+
+**Status:** Proposed (2026-06-12). Issue #16. **Draft for review — no code yet.**
+
+**Context / requirements.** The reader is multi-page: a list of compact cards (`/`), then a
+navigation to a separate detail page (`/v/{id}`) for the full summary + the docked chat (ADR-027).
+It feels less fluid than a single integrated view. We want the full summary AND the per-video
+Q&A to open **in place in the list**, no page hop. Requirements:
+- R1: clicking a summarized card expands it in place to the full summary (summary/highlights/
+  takeaways) + the chat dock; a collapse returns it to the compact card.
+- R2: **no SPA framework** — stay HTMX + Templ (ADR-003); reuse existing fragments, not a rewrite.
+- R3: **progressive enhancement** — with JS off, the card link still navigates to `/v/{id}`
+  (the detail page stays as the no-JS + deep-link surface). Nothing becomes JS-only.
+- R4: only **summarized** cards expand; pending/rate-limited/no-caption cards keep their current
+  footer behaviour (Summarize button, waiting/none states).
+- R5: chat inside an expanded card works exactly as on the detail page (reuse `chatReveal`/
+  `chatSection` + the existing `/v/{id}/chat` endpoints, unchanged).
+
+**Decision / architecture.**
+1. **Reuse the existing fragments.** `summaryBody(r)` and `chatReveal(videoID)` already exist and
+   render the detail page; a new `expandedCard(r, chatEnabled)` composes the compact header + a
+   collapse control + `summaryBody` + `chatReveal`. `DetailPage` is refactored to also compose
+   `summaryBody` so the two never drift (DRY).
+2. **Two fragment endpoints** (mirroring the existing list/status HTMX fragment pattern):
+   `GET /v/{videoId}/expand` → `expandedCard`; collapse reuses the existing compact `VideoCard`
+   via `GET /v/{videoId}/card`. Both are list-card `<li>` fragments with the SAME `id`
+   (`video-{id}`), swapped `outerHTML` — same mechanism as `processingCard`/`VideoCard` today.
+3. **The compact card's title/"Read" affordance** becomes `hx-get=/v/{id}/expand`,
+   `hx-target=#video-{id}`, `hx-swap=outerHTML`, with `href=/v/{id}` as the no-JS fallback (R3).
+   The expanded card's collapse control is the inverse (`hx-get=/v/{id}/card`).
+4. **Only when `r.Summarized`** does the expand affordance render (R4); the other states are
+   unchanged.
+5. **v1 does NOT push the URL** (`hx-push-url`) — expand/collapse is ephemeral list UI state; the
+   detail page remains the deep-link/shareable URL. Deep-linking the open state via `hx-push-url`
+   is noted as a later option (needs list-state restore on back).
+
+**Out of scope / later.** URL push / deep-linkable open state; the visual refresh (#17) — though
+the expanded-card markup is where #17's TUI/charm styling will land, so they pair.
+
+**Reversibility.** Additive: two fragment endpoints + one templ + an affordance swap on the
+compact card. Removing the affordance reverts to plain list→detail navigation; the detail page is
+untouched. No schema change.
+
+**Next steps (gated):** on approval → BDD (`docs/use-cases/inline_expand.feature` + coverage
+map) → TDD → implement → SemVer + docs.
+
+---
+
 ## Rejected alternatives
 
 Approaches considered during the 2026-06-02 planning + grill session and **deliberately not