feat(store): config table for instance-global app-config overrides

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/store/schema.sql

@@ -251,3 +251,14 @@ CREATE TABLE sync_runs (
     status             TEXT NOT NULL CHECK(status IN ('running','success','error')),
     error_message      TEXT
 );
+
+-- Application configuration: instance-global key/value overrides, shared
+-- by every user -- deliberately the one table with no user_id, because
+-- application configuration is common to all users by definition. Stores
+-- overrides only; defaults live in internal/config's app-key registry.
+-- Cold: read once at startup, a change applies on the next backend restart.
+CREATE TABLE config (
+    key        TEXT PRIMARY KEY,
+    value      TEXT NOT NULL,
+    updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+);