Add Discord outbound posting (#14)

Stacks on #13. Adds discord_post_links and a subscriber that posts roots into a public thread, replies into that thread, and edits the linked Discord message. Unset Discord env leaves the site unchanged.

Reviewed-on: #14
Co-authored-by: codegirl-007 <s.raide@gmail.com>

schema.sql

@@ -65,6 +65,17 @@ CREATE TABLE IF NOT EXISTS post_votes (
 CREATE INDEX IF NOT EXISTS idx_post_votes_post_id
   ON post_votes(post_id);
 
+CREATE TABLE IF NOT EXISTS discord_post_links (
+  post_id TEXT PRIMARY KEY REFERENCES posts(id) ON DELETE CASCADE,
+  discord_message_id TEXT NOT NULL UNIQUE,
+  discord_thread_id TEXT NOT NULL DEFAULT '',
+  created_at TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS discord_post_links_thread_uidx
+  ON discord_post_links (discord_thread_id)
+  WHERE discord_thread_id <> '';
+
 CREATE TABLE IF NOT EXISTS sessions (
   token TEXT PRIMARY KEY,
   data BYTEA NOT NULL,