feat(bots): built-in bot runner + steamfree from UI

- BotRunner: server-side goroutine manager for built-in bot types
- steamfree bot embedded in server (polls Steam API, posts free games)
- bot_type + config JSONB columns on bots table
- Create/Update/Delete handlers manage runner lifecycle
- GET /bots/types returns registered bot types
- BotManager: type selector dropdown + config fields on create
- No SSH needed: create a 'Steam Free Games' bot from /bots/manage

internal/db/db.go

@@ -569,5 +569,10 @@ CREATE INDEX IF NOT EXISTS idx_feature_request_votes_fr ON feature_request_votes
 -- Bot messages: track which bot authored a message
 ALTER TABLE messages ADD COLUMN IF NOT EXISTS bot_id UUID REFERENCES bots(id) ON DELETE SET NULL;
 CREATE INDEX IF NOT EXISTS idx_messages_bot ON messages(bot_id) WHERE bot_id IS NOT NULL;
+
+-- Built-in bot types: bot_type + config for server-managed bots
+ALTER TABLE bots ADD COLUMN IF NOT EXISTS bot_type VARCHAR(32) DEFAULT '';
+ALTER TABLE bots ADD COLUMN IF NOT EXISTS config JSONB DEFAULT '{}';
+CREATE INDEX IF NOT EXISTS idx_bots_type ON bots(bot_type) WHERE bot_type != '';
 `