feat(phase7): per-channel notification settings and read receipts

Phase 7.1 — Per-channel notification settings
- New notification_settings table (user_id, channel_id, level)
- GET/PUT/DELETE handlers under /channels/{channelID}/notifications
- Push dispatch and @mention loops filtered by notification level
- Frontend: bell icon per channel cycling all/mentions/none

Phase 7.4 — Read receipts
- New read_states table (user_id, channel_id, last_read_message_id)
- PUT /channels/{channelID}/read and GET /users/me/read-states
- Auto-mark-read when messages load in ChatArea
- Unread dot for channels never opened

Also: favicon/icon refresh in index.html

internal/db/db.go

@@ -215,6 +215,23 @@ CREATE TABLE IF NOT EXISTS webauthn_credentials (
 CREATE INDEX IF NOT EXISTS idx_push_subscriptions_user ON push_subscriptions(user_id);
 CREATE INDEX IF NOT EXISTS idx_webauthn_credentials_user ON webauthn_credentials(user_id);
 
+-- Per-channel notification settings
+CREATE TABLE IF NOT EXISTS notification_settings (
+	user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+	channel_id UUID NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+	level VARCHAR(16) NOT NULL DEFAULT 'all',
+	PRIMARY KEY (user_id, channel_id)
+);
+
+-- Read receipts (per-user, per-channel)
+CREATE TABLE IF NOT EXISTS read_states (
+	user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+	channel_id UUID NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+	last_read_message_id UUID NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
+	updated_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
+	PRIMARY KEY (user_id, channel_id)
+);
+
 -- Direct message conversations
 CREATE TABLE IF NOT EXISTS conversations (
 	id UUID PRIMARY KEY DEFAULT gen_random_uuid(),