feat(web): chat with an uploaded transcript, reusing the ADR-027 path (ADR-033)

GET/POST /uploads/{id}/chat, mounted only when both an Uploader and a Chatter
are wired. The row comes from the user-scoped private store, so another user's
upload is a 404 before any model is involved. The owner's transcript goes to
the SAME Chatter with App.UploadChatMaxChars instead of the 18,000-char caption
default. The chat panel is shared, not forked: chatView gains PostURL/Subject
(empty = a YouTube video, so existing callers are unchanged). The done card
links to chat only when chat is wired.

Mutation-checked: dropping the upload budget, ignoring the done-status guard,
or scoping the lookup to the wrong user each turn tests red.

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

internal/web/uploads.go

@@ -9,6 +9,9 @@ import (
 	"strings"
 	"unicode/utf8"
 
+	"github.com/a-h/templ"
+
+	"git.d-ma.be/mathias/tapir/internal/adapters/chat"
 	"git.d-ma.be/mathias/tapir/internal/adapters/store"
 	"git.d-ma.be/mathias/tapir/internal/domain"
 	"git.d-ma.be/mathias/tapir/internal/uploads"
@@ -129,7 +132,7 @@ func (a *App) handleUploadDetail(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	a.render(w, r, UploadPage(*u))
+	a.render(w, r, UploadPage(*u, a.Chat != nil))
 }
 
 // handleUploadStatus renders the status fragment HTMX polls. Unfinished states
@@ -140,7 +143,7 @@ func (a *App) handleUploadStatus(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	a.render(w, r, uploadStatus(*u))
+	a.render(w, r, uploadStatus(*u, a.Chat != nil))
 }
 
 func (a *App) loadOwnedUpload(w http.ResponseWriter, r *http.Request) (*domain.UploadedTranscript, bool) {
@@ -189,3 +192,79 @@ func uploadFailureMessage(k domain.UploadErrorKind) string {
 		return "This upload failed. Please upload it again."
 	}
 }
+
+// handleUploadChat renders the chat page for a FINISHED upload (GET). The row is
+// read through the user-scoped store, so another user's id is a 404 before any
+// model is involved. An unfinished upload has no transcript to ask about yet, so
+// it goes back to its status page.
+func (a *App) handleUploadChat(w http.ResponseWriter, r *http.Request) {
+	u, ok := a.loadOwnedUpload(w, r)
+	if !ok {
+		return
+	}
+	if u.Status != domain.UploadDone {
+		http.Redirect(w, r, "/uploads/"+u.ID, http.StatusSeeOther)
+		return
+	}
+	a.render(w, r, UploadChatPage(*u, a.uploadChatView(*u, a.Chat.DefaultModel(""), nil, false, "")))
+}
+
+// handleUploadChatMessage answers one question against the owner's uploaded
+// transcript (POST), reusing the ADR-027 Chatter. Nothing is persisted: the prior
+// turns ride in the form. The transcript goes to the model with the UPLOAD budget,
+// not the caption default. A model failure is inline, not a 500.
+func (a *App) handleUploadChatMessage(w http.ResponseWriter, r *http.Request) {
+	u, ok := a.loadOwnedUpload(w, r)
+	if !ok {
+		return
+	}
+	if u.Status != domain.UploadDone {
+		http.Redirect(w, r, "/uploads/"+u.ID, http.StatusSeeOther)
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "bad form", http.StatusBadRequest)
+		return
+	}
+	model := a.resolveModel(r.FormValue("model"), "")
+	history := parseHistory(r.Form["hq"], r.Form["ha"])
+	question := strings.TrimSpace(r.FormValue("question"))
+
+	view := a.uploadChatView(*u, model, history, false, "")
+	if question != "" {
+		reply, err := a.Chat.Answer(r.Context(), chat.Request{
+			Model:      model,
+			Transcript: u.Content,
+			History:    history,
+			Question:   question,
+			MaxChars:   a.UploadChatMaxChars,
+		})
+		if err != nil {
+			a.logger().Error("upload chat answer", "upload", u.ID, "model", model, "err", err)
+			view.Error = "That model couldn't answer just now. Try again, or switch models."
+		} else {
+			view.History = appendTurn(history, chat.Turn{Question: question, Answer: reply.Answer})
+			view.Truncated = reply.Truncated
+		}
+	}
+	if isHTMX(r) {
+		a.render(w, r, chatPanel(view))
+		return
+	}
+	a.render(w, r, UploadChatPage(*u, view))
+}
+
+func (a *App) uploadChatView(u domain.UploadedTranscript, model string, history []chat.Turn, truncated bool, errMsg string) chatView {
+	return chatView{
+		VideoID:   u.ID,
+		Title:     u.Title,
+		Available: true,
+		Models:    a.Chat.Models(),
+		Selected:  model,
+		History:   history,
+		Truncated: truncated,
+		Error:     errMsg,
+		PostURL:   templ.SafeURL("/uploads/" + u.ID + "/chat"),
+		Subject:   "recording",
+	}
+}