feat(modules): config loader refactor — YAML → DB → env pipeline

Add ModulesConfig and ModuleToggle types to config.go for explicit
module enable/disable in YAML.

Add LoadState() that merges state from three sources:
1. Backward-compat YAML fields (auth.enabled, odoo.enabled)
2. Explicit modules.* YAML toggles (override compat)
3. Database module_state table (highest precedence)

Validates dependency chain after loading. 5 loader tests.

Ref #95

internal/config/config.go

@@ -18,6 +18,25 @@ type Config struct {
 	Odoo     OdooConfig     `yaml:"odoo"`
 	Auth     AuthConfig     `yaml:"auth"`
 	Jobs     JobsConfig     `yaml:"jobs"`
+	Modules  ModulesConfig  `yaml:"modules"`
+}
+
+// ModulesConfig holds explicit enable/disable toggles for optional modules.
+// A nil pointer means "use the module's default state".
+type ModulesConfig struct {
+	Auth     *ModuleToggle `yaml:"auth"`
+	Projects *ModuleToggle `yaml:"projects"`
+	Audit    *ModuleToggle `yaml:"audit"`
+	Odoo     *ModuleToggle `yaml:"odoo"`
+	FreeCAD  *ModuleToggle `yaml:"freecad"`
+	Jobs     *ModuleToggle `yaml:"jobs"`
+	DAG      *ModuleToggle `yaml:"dag"`
+}
+
+// ModuleToggle holds an optional enabled flag. The pointer allows
+// distinguishing "not set" (nil) from "explicitly false".
+type ModuleToggle struct {
+	Enabled *bool `yaml:"enabled"`
 }
 
 // AuthConfig holds authentication and authorization settings.