* admin: configure maintenance tasks via admin.toml
Maintenance task settings could only be edited in the admin UI and live
under <dataDir>/conf, so they silently reverted to defaults whenever the
data directory was recreated. An optional admin.toml now declares vacuum,
balance, and erasure coding settings; keys set there are written through
to the persisted task configs at every startup, overriding UI edits, so
the configuration stays declarative. Generate an example with
"weed scaffold -config=admin".
* vacuum: round min volume age up to whole hours
MinVolumeAgeSeconds was truncated by integer division when converted to
the hour-granular protobuf field, so a sub-hour setting silently became
0 and disabled the age guard.
* admin: split and normalize preferred_tags from admin.toml
A comma-separated string, as set via environment variable, came through
viper as a single slice element. Split on commas and reuse
util.NormalizeTagList, matching the plugin config path.
* scaffold: clarify admin.toml wording
