feat(discussions): questions on published studysets, answered by learners

A learner asks about a published studyset, a topic in it, or one card,
note or step of a release, and others answer, as on Stack Exchange
(ADR 0013): questions and answers in the app's editor without images,
sounds or videos, comments of up to 600 characters under the question
and each answer. There are no votes: the asker accepts an answer, which
comes first, then the oldest; the author's posts are marked. The
instance reads from git's file list which topics a question's target
sits in, so a topic lists what is asked within it and below it.

Posts live in two server-only tables, written through endpoints, so the
synced tables stay as they are and v0.17.0 keeps syncing. Everyone
signed in reads; writing needs an identity and a public profile; the
studyset's author hides posts and blocks people with feedback's blocks;
a withdrawn studyset takes no new questions; anyone writes at most 30
posts an hour. A post with others' replies under it is emptied when
deleted, else deleted; deletePerson deletes what a person wrote the same
way, and a published studyset's deletion takes its discussions with it.

The migration adds Serverpod's gen_random_uuid_v7(), discussion_post
and discussion_seen with their indexes and cascading foreign keys, and
changes nothing else.

In the app, discussions open from a copy's studyset or topic menu, a
studied card's "Ask about this", Explore's summary and the Publish
sheet; the Profile tab's inbox lists those the person follows, and its
count includes what is new in them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -2051,11 +2051,78 @@
   "feedbackUnreachable": "Reports and suggestions can't be reached right now.",
   "feedbackRetry": "Retry",
   "inboxTitle": "Inbox",
-  "inboxIntro": "Reports and suggestions on your published studysets, and those you wrote.",
+  "inboxIntro": "Questions and answers, reports and suggestions on your published studysets, and those you wrote in.",
   "inboxEmpty": "Nothing yet.",
   "studysetFeedback": "Reports and suggestions",
   "studysetFeedbackEmpty": "Nobody has reported or suggested anything yet.",
   "reportToAuthor": "Report to the author",
+  "discussionsTitle": "Discussions",
+  "discussionsSearch": "Search questions",
+  "discussionsAsk": "Ask a question",
+  "discussionsEmpty": "No questions yet",
+  "discussionsEmptyHint": "Ask the first: about the studyset, a topic or one of its flashcards.",
+  "discussionsNoMatch": "No question's title says that.",
+  "discussionsMore": "Show more",
+  "discussionsUnreachable": "The discussions can't be reached right now.",
+  "discussionsAnswers": "{count, plural, =0{No answers} =1{1 answer} other{{count} answers}}",
+  "@discussionsAnswers": {"placeholders": {"count": {"type": "int"}}},
+  "discussionsBy": "{name} · {date}",
+  "@discussionsBy": {"placeholders": {"name": {"type": "String"}, "date": {"type": "String"}}},
+  "discussionsSomeone": "Someone",
+  "discussionsDeletedQuestion": "Deleted question",
+  "discussionsHidden": "Hidden",
+  "discussionsAboutCard": "About a flashcard",
+  "discussionsAboutNote": "About a note",
+  "discussionsAboutStep": "About a step",
+  "discussionsAccepted": "Accepted",
+  "discussionsOn": "On {title}",
+  "@discussionsOn": {"placeholders": {"title": {"type": "String"}}},
+  "discussionRefusalNoIdentity": "To ask and answer, sign in with an identity in Profile: posts show who wrote them.",
+  "discussionRefusalNoProfile": "To ask and answer, make a public profile in Profile: posts show who wrote them.",
+  "discussionRefusalBlocked": "The author of this studyset keeps you from writing on their studysets.",
+  "discussionRefusalWithdrawn": "The author took this studyset off Explore: what is here stays, but nothing new is asked.",
+  "discussionRefusalNotInRelease": "That isn't in the studyset's release any more.",
+  "discussionRefusalEmpty": "Write something first.",
+  "discussionRefusalTooLong": "That's longer than allowed.",
+  "discussionRefusalTooMany": "You've written a lot in the last hour. Try again a little later.",
+  "discussionRefusalWithFiles": "Images and sounds can't be posted: your files are yours alone, so nobody else would see them.",
+  "discussionFailed": "Couldn't reach the instance. Try again once you are online.",
+  "discussionTitle": "Question",
+  "discussionMissing": "This question isn't there any more.",
+  "discussionDeleted": "This was deleted.",
+  "discussionEdited": "edited",
+  "discussionWriteAnswer": "Write an answer",
+  "discussionYourAnswer": "Your answer",
+  "discussionEditHeading": "Edit",
+  "discussionTitleField": "Title",
+  "discussionTitleHint": "What do you want to know?",
+  "discussionPost": "Post",
+  "discussionComment": "Add a comment",
+  "discussionCommentHint": "A short remark",
+  "discussionAccept": "Accept",
+  "discussionUnaccept": "Take back",
+  "discussionMenu": "More",
+  "discussionMenuEdit": "Edit",
+  "discussionMenuDelete": "Delete",
+  "discussionMenuHide": "Hide",
+  "discussionMenuShow": "Show again",
+  "discussionMenuBlock": "Block {name}",
+  "@discussionMenuBlock": {"placeholders": {"name": {"type": "String"}}},
+  "discussionDeleteTitle": "Delete this?",
+  "discussionDeleteBody": "What others replied under it stays, without your text and name.",
+  "discussionAnswered": "Answer posted.",
+  "discussionCommented": "Comment posted.",
+  "discussionSaved": "Saved.",
+  "discussionDeletedDone": "Deleted.",
+  "discussionAcceptedDone": "Answer accepted.",
+  "discussionUnacceptedDone": "No answer accepted now.",
+  "discussionHiddenDone": "Hidden: only its writer and you see it.",
+  "discussionShownDone": "Shown again.",
+  "discussionBlockedDone": "Blocked: what they write on your studysets shows to them alone.",
+  "discussionAskAbout": "Ask about this",
+  "inboxDiscussions": "Discussions",
+  "inboxFeedback": "Reports and suggestions",
+  "inboxDiscussionsEmpty": "No discussions yet.",
   "cardReportToAuthor": "Send to the author too",
   "cardReportToAuthorHint": "Public: everyone learning it reads it, with your public profile.",
   "feedbackReportSent": "Sent to the author.",