refactor: drop server_sessions table

Add migration to remove server consultation sessions table.
Update schema.sql to remove table definition.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/012_drop_server_sessions.sql

@@ -0,0 +1,5 @@
+-- Drop server consultation sessions table
+-- This table was used for server recommendation consultation feature which is now removed
+
+DROP TABLE IF EXISTS server_sessions;
+DROP INDEX IF EXISTS idx_server_sessions_expires;