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/modules/loader.go

@@ -0,0 +1,84 @@
+package modules
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/kindredsystems/silo/internal/config"
+)
+
+// LoadState applies module state from config YAML and database overrides.
+//
+// Precedence (highest wins):
+//  1. Database module_state table
+//  2. YAML modules.* toggles
+//  3. Backward-compat YAML fields (auth.enabled, odoo.enabled)
+//  4. Module defaults (set by NewRegistry)
+func LoadState(r *Registry, cfg *config.Config, pool *pgxpool.Pool) error {
+	// Step 1: Apply backward-compat top-level YAML fields.
+	// auth.enabled and odoo.enabled existed before the modules section.
+	// Only apply if the new modules.* section doesn't override them.
+	if cfg.Modules.Auth == nil {
+		r.setEnabledUnchecked(Auth, cfg.Auth.Enabled)
+	}
+	if cfg.Modules.Odoo == nil {
+		r.setEnabledUnchecked(Odoo, cfg.Odoo.Enabled)
+	}
+
+	// Step 2: Apply explicit modules.* YAML toggles (override defaults + compat).
+	applyToggle(r, Auth, cfg.Modules.Auth)
+	applyToggle(r, Projects, cfg.Modules.Projects)
+	applyToggle(r, Audit, cfg.Modules.Audit)
+	applyToggle(r, Odoo, cfg.Modules.Odoo)
+	applyToggle(r, FreeCAD, cfg.Modules.FreeCAD)
+	applyToggle(r, Jobs, cfg.Modules.Jobs)
+	applyToggle(r, DAG, cfg.Modules.DAG)
+
+	// Step 3: Apply database overrides (highest precedence).
+	if pool != nil {
+		if err := loadFromDB(r, pool); err != nil {
+			return err
+		}
+	}
+
+	// Step 4: Validate the final state.
+	return r.ValidateDependencies()
+}
+
+// applyToggle sets a module's state from a YAML ModuleToggle if present.
+func applyToggle(r *Registry, id string, toggle *config.ModuleToggle) {
+	if toggle == nil || toggle.Enabled == nil {
+		return
+	}
+	r.setEnabledUnchecked(id, *toggle.Enabled)
+}
+
+// setEnabledUnchecked sets module state without dependency validation.
+// Used during loading when the full state is being assembled incrementally.
+func (r *Registry) setEnabledUnchecked(id string, enabled bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if m, ok := r.modules[id]; ok && !m.Required {
+		m.enabled = enabled
+	}
+}
+
+// loadFromDB reads module_state rows and applies them to the registry.
+func loadFromDB(r *Registry, pool *pgxpool.Pool) error {
+	rows, err := pool.Query(context.Background(),
+		`SELECT module_id, enabled FROM module_state`)
+	if err != nil {
+		return err
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var id string
+		var enabled bool
+		if err := rows.Scan(&id, &enabled); err != nil {
+			return err
+		}
+		r.setEnabledUnchecked(id, enabled)
+	}
+	return rows.Err()
+}