Rename "channels" to "conversations."

The term "channel" for a conversational container has a long and storied history, but is mostly evocative of IRC and of other, ah, "nerd-centric" services. It does show up in more widespread contexts: Discord and Slack both refer to their primary conversational containers as "channels," for example. However, I think it's unnecessary jargon, and I'd like to do away with it.

To that end, this change pervasively changes one term to the other wherever it appears, with the following exceptions:

* A `channel` concept (unrelated to conversations) is also provided by an external library; we can't and shouldn't try to rename that.
* The code to deal with the `pilcrow:channelData` and `pilcrow:lastActiveChannel` local storage properties is still present, to migrate existing data to new keys. It will be removed in a later change.

This is a **breaking API change**. As we are not yet managing any API compatibility promises, this is formally not an issue, but it is something to be aware of practically. The major API changes are:

* Paths beginning with `/api/channels` are now under `/api/conversations`, without other modifications.
* Fields labelled with `channel…` terms are now labelled with `conversation…` terms. For example, a `message` `sent` event is now sent to a `conversation`, not a `channel`.

This is also a **breaking UI change**. Specifically, any saved paths for `/ch/CHANNELID` will now lead to a 404. The corresponding paths are `/c/CONVERSATIONID`. While I've made an effort to migrate the location of stored data, I have not tried to provide adapters to fix this specific issue, because the disruption is short-lived and very easily addressed by opening a channel in the client UI.

This change is obnoxiously large and difficult to review, for which I apologize. If this shows up in `git annotate`, please forgive me. These kinds of renamings are hard to carry out without a major disruption, especially when the concept ("channel" in this case) is used so pervasively throughout the system.

I think it's worth making this change that pervasively so that we don't have an indefinitely-long tail of "well, it's a conversation in the docs, but the table is called `channel` for historical reasons" type issues.

Merges conversations-not-channels into main.

1 files changed, 153 insertions, 0 deletions

diff --git a/migrations/20250624003106_rename_channel_to_conversation.sql b/migrations/20250624003106_rename_channel_to_conversation.sql
new file mode 100644
index 0000000..753d42b
--- /dev/null
+++ b/migrations/20250624003106_rename_channel_to_conversation.sql
@@ -0,0 +1,153 @@
+alter table message
+    rename to old_message;
+
+alter table message_deleted
+    rename to old_message_deleted;
+
+create table conversation (
+    id text
+        not null
+        primary key,
+    created_sequence bigint
+        unique
+        not null,
+    created_at text
+        not null,
+    last_sequence bigint
+        not null
+);
+
+insert into
+    conversation (id, created_sequence, created_at, last_sequence)
+select
+    id,
+    created_sequence,
+    created_at,
+    last_sequence
+from
+    channel;
+
+create table conversation_name (
+    id text
+        not null
+        primary key
+        references conversation (id),
+    display_name
+        not null,
+    canonical_name
+        not null
+        unique
+);
+
+insert into
+    conversation_name (id, display_name, canonical_name)
+select
+    id,
+    display_name,
+    canonical_name
+from
+    channel_name;
+
+create table conversation_deleted (
+    id text
+        not null
+        primary key
+        references conversation (id),
+    deleted_sequence bigint
+        unique
+        not null,
+    deleted_at text
+        not null
+);
+
+insert into
+    conversation_deleted (id, deleted_sequence, deleted_at)
+select
+    id,
+    deleted_sequence,
+    deleted_at
+from
+    channel_deleted;
+
+create table message (
+    id text
+        not null
+        primary key,
+    conversation text
+        not null
+        references conversation (id),
+    sender text
+        not null
+        references user (id),
+    sent_sequence bigint
+        unique
+        not null,
+    sent_at text
+        not null,
+    body text
+        null,
+    last_sequence bigint
+        not null
+);
+
+insert into
+    message (id, conversation, sender, sent_sequence, sent_at, body, last_sequence)
+select
+    id,
+    channel,
+    sender,
+    sent_sequence,
+    sent_at,
+    body,
+    last_sequence
+from
+    old_message;
+
+create table message_deleted (
+    id text
+        not null
+        primary key
+        references message (id),
+    deleted_sequence bigint
+        unique
+        not null,
+    deleted_at text
+        not null
+
+);
+
+insert into
+    message_deleted (id, deleted_sequence, deleted_at)
+select
+    id,
+    deleted_sequence,
+    deleted_at
+from
+    old_message_deleted;
+
+drop table old_message_deleted;
+drop table old_message;
+drop table channel_deleted;
+drop table channel_name;
+drop table channel;
+
+create index conversation_created_sequence
+    on conversation (created_sequence);
+
+create index conversation_created_at
+    on conversation (created_at);
+
+create index conversation_last_sequence
+    on conversation (last_sequence);
+
+create index message_deleted_deleted_at
+    on message_deleted (deleted_at);
+
+create index message_sent_at
+    on message (sent_at);
+
+create index message_conversation
+    on message (conversation);
+
+create index message_last_sequence
+    on message (last_sequence);