R-492: delete the always-empty cfg.Paths.HDDPath (field, env binding, every fallback)

Readers (report builder, health check, /api/system, web primaryHDDPath, metrics collector,
AutoDiscoverStoragePaths' fallback parameter) now use the storage registry only. An old
controller.yaml still carrying paths.hdd_path keeps loading (non-strict YAML), pinned by
TestR492_OldConfigWithHDDPathStillLoads.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/settings/storage_discovery_test.go

@@ -30,7 +30,6 @@ func TestAutoDiscoverStoragePaths_Additive(t *testing.T) {
 		name        string
 		existing    []StoragePath
 		discovered  []string
-		fallback    string
 		wantPaths   []string // expected Path set after discovery (order-insensitive)
 		wantNewPath string   // a path expected to be newly added (may be "")
 		// assertions run against the resulting registry
@@ -101,17 +100,6 @@ func TestAutoDiscoverStoragePaths_Additive(t *testing.T) {
 				}
 			},
 		},
-		{
-			name: "fallback path registered when missing",
-			existing: []StoragePath{
-				{Path: "/mnt/felhom-usb", Label: "x", IsDefault: true, Schedulable: true, AddedAt: "2026-01-01T00:00:00Z"},
-			},
-			discovered:  nil,
-			fallback:    "/mnt/legacy_hdd",
-			wantPaths:   []string{"/mnt/felhom-usb", "/mnt/legacy_hdd"},
-			wantNewPath: "/mnt/legacy_hdd",
-			check:       nil,
-		},
 	}
 
 	for _, tc := range tests {
@@ -119,7 +107,7 @@ func TestAutoDiscoverStoragePaths_Additive(t *testing.T) {
 			s := newTestSettings(t, cloneStoragePaths(tc.existing))
 			before := cloneStoragePaths(tc.existing)
 
-			s.AutoDiscoverStoragePaths(tc.discovered, tc.fallback, logger)
+			s.AutoDiscoverStoragePaths(tc.discovered, logger)
 
 			// Normalize with filepath.Clean so comparisons hold on both Linux (the deploy
 			// target) and Windows (the dev machine), where Clean uses backslashes.
@@ -158,7 +146,7 @@ func TestAutoDiscoverStoragePaths_DecommissionedNotReactivated(t *testing.T) {
 	before := cloneStoragePaths(existing)
 
 	// A deployed app still points at the decommissioned drive.
-	s.AutoDiscoverStoragePaths([]string{"/mnt/old_hdd", "/mnt/felhom-usb"}, "", logger)
+	s.AutoDiscoverStoragePaths([]string{"/mnt/old_hdd", "/mnt/felhom-usb"}, logger)
 
 	if len(s.StoragePaths) != 2 {
 		t.Fatalf("path count changed: got %d want 2 (%v)", len(s.StoragePaths), pathList(s))