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/views.templ

@@ -583,7 +583,7 @@ templ chatPanel(v chatView) {
 				</div>
 			}
 			if v.Truncated {
-				<p class="chat-note muted">Working from a bounded portion of a long transcript — answers about the end of the video may be incomplete.</p>
+				<p class="chat-note muted">Working from a bounded portion of a long transcript — answers about the end of the { v.subject() } may be incomplete.</p>
 			}
 			if v.Error != "" {
 				<p class="chat-error" role="alert">{ v.Error }</p>
@@ -591,8 +591,8 @@ templ chatPanel(v chatView) {
 			<form
 				class="chat-form"
 				method="post"
-				action={ chatURL(v.VideoID) }
-				hx-post={ string(chatURL(v.VideoID)) }
+				action={ v.postURL() }
+				hx-post={ string(v.postURL()) }
 				hx-target="#chat-panel"
 				hx-swap="outerHTML"
 			>
@@ -609,7 +609,7 @@ templ chatPanel(v chatView) {
 					</select>
 					<span class="chat-model-hint muted">Switch models to compare answers on the same transcript.</span>
 				</label>
-				<textarea name="question" rows="3" placeholder="Ask a question about this video…" required aria-label="Your question"></textarea>
+				<textarea name="question" rows="3" placeholder={ "Ask a question about this " + v.subject() + "…" } required aria-label="Your question"></textarea>
 				<button type="submit" class="btn">Ask</button>
 				<span class="htmx-indicator chat-thinking">thinking…</span>
 			</form>