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

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

backend/internal/store/config.go

@@ -0,0 +1,39 @@
+package store
+
+import (
+	"context"
+	"fmt"
+)
+
+// ConfigValues returns every application-configuration override row as
+// key -> value. An absent key means its default (from internal/config's
+// app-key registry) is in effect -- the table stores overrides only.
+func (db *DB) ConfigValues(ctx context.Context) (map[string]string, error) {
+	rows, err := db.QueryContext(ctx, `SELECT key, value FROM config`)
+	if err != nil {
+		return nil, fmt.Errorf("config values: %w", err)
+	}
+	defer rows.Close()
+
+	values := map[string]string{}
+	for rows.Next() {
+		var k, v string
+		if err := rows.Scan(&k, &v); err != nil {
+			return nil, fmt.Errorf("scan config row: %w", err)
+		}
+		values[k] = v
+	}
+	return values, rows.Err()
+}
+
+// SetConfigValue upserts one override row. Key validation is the caller's
+// job (config.ValidateAppValue) -- the store stays a dumb K/V layer.
+func (db *DB) SetConfigValue(ctx context.Context, key, value string) error {
+	if _, err := db.ExecContext(ctx, `
+		INSERT INTO config (key, value, updated_at) VALUES (?, ?, datetime('now'))
+		ON CONFLICT(key) DO UPDATE SET value = excluded.value, updated_at = excluded.updated_at`,
+		key, value); err != nil {
+		return fmt.Errorf("set config %q: %w", key, err)
+	}
+	return nil
+}

backend/internal/store/config_test.go

@@ -0,0 +1,35 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestConfigValues_EmptyThenUpsertOverwrites(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	values, err := db.ConfigValues(ctx)
+	if err != nil {
+		t.Fatalf("ConfigValues (empty): %v", err)
+	}
+	if len(values) != 0 {
+		t.Fatalf("expected no overrides in a fresh DB, got %v", values)
+	}
+
+	if err := db.SetConfigValue(ctx, "session.duration", "168"); err != nil {
+		t.Fatalf("SetConfigValue: %v", err)
+	}
+	// Same key again: upsert must overwrite, not error or duplicate.
+	if err := db.SetConfigValue(ctx, "session.duration", "24"); err != nil {
+		t.Fatalf("SetConfigValue (overwrite): %v", err)
+	}
+
+	values, err = db.ConfigValues(ctx)
+	if err != nil {
+		t.Fatalf("ConfigValues: %v", err)
+	}
+	if len(values) != 1 || values["session.duration"] != "24" {
+		t.Fatalf("expected {session.duration: 24}, got %v", values)
+	}
+}

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'))
+);