Bind history to the recipient session and channel, not the name (closes #148)

History was selected by msg_to, so whoever next held a released nick, or
created a deleted channel again under its old name, could read the
earlier holder's messages. Each message row now records the session a
DM or INVITE was sent to (to_session_id) and the channel a channel
message was sent to (to_channel_id), and GET /api/v1/history selects by
those ids. A session's DM history now also follows it across nick
changes.

Model: opus-5-5

internal/db/schema/001_initial.sql

@@ -85,19 +85,25 @@ CREATE TABLE IF NOT EXISTS channel_members (
     UNIQUE(channel_id, session_id)
 );
 
--- Messages: IRC envelope format
+-- Messages: IRC envelope format. to_session_id is the session a DM
+-- or INVITE was sent to, to_channel_id the channel a channel message
+-- was sent to. History is selected by these ids because nicks and
+-- channel names are reused once released, and ids are not.
 CREATE TABLE IF NOT EXISTS messages (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     uuid TEXT NOT NULL UNIQUE,
     command TEXT NOT NULL DEFAULT 'PRIVMSG',
     msg_from TEXT NOT NULL DEFAULT '',
     msg_to TEXT NOT NULL DEFAULT '',
+    to_session_id INTEGER REFERENCES sessions(id) ON DELETE SET NULL,
+    to_channel_id INTEGER REFERENCES channels(id) ON DELETE SET NULL,
     params TEXT NOT NULL DEFAULT '[]',
     body TEXT NOT NULL DEFAULT '[]',
     meta TEXT NOT NULL DEFAULT '{}',
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP
 );
-CREATE INDEX IF NOT EXISTS idx_messages_to_id ON messages(msg_to, id);
+CREATE INDEX IF NOT EXISTS idx_messages_to_session ON messages(to_session_id, id);
+CREATE INDEX IF NOT EXISTS idx_messages_to_channel ON messages(to_channel_id, id);
 CREATE INDEX IF NOT EXISTS idx_messages_created ON messages(created_at);
 
 -- Spent hashcash tokens for replay prevention (1-year TTL)