feat: structured NFS options with auto-fsid and manual watched mounts

NFS exports: replace plain-text options string with typed booleans
(read_only, async, root_squash, subtree_check) + advanced JSON blob.
fsid is auto-generated via crypto/rand with collision retry and is
never user-settable. Breaking API change (options field removed).

Dashboard: filter disks to manual+samba+nfs sources only; no more
auto-discovery of all /proc/mounts entries.

Settings: new 'Puntos de montaje vigilados' card with add/remove
for manual mount points. AllowedRoots validation applied.

Version bump: 0.1.10 -> 0.2.0

internal/db/models.go

@@ -59,6 +59,12 @@ type NFSAdvanced struct {
 	Crossmnt   bool `json:"crossmnt"`
 }
 
+type WatchedMount struct {
+	ID        int64     `json:"id"`
+	Path      string    `json:"path"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
 type DirtyModule struct {
 	Module   string    `json:"module"`
 	MarkedAt time.Time `json:"marked_at"`