#2516 Docs: document server token GC and --token-gc-interval / --token-gc-grace-period in serve-flags.md

swamp-club#2413 makes swamp serve garbage-collect server tokens in every auth mode. The manual page content/manual/reference/swamp-serve/serve-flags.md needs updating.

New settings

• Both take whole seconds or larger units (30s, 1h, 1d).
• The interval is capped at the maximum safe timer duration (about 24.8 days).
• Both flags are also accepted by swamp serve daemon enable.

Behaviour to describe

• The first sweep runs just after boot, then one runs every interval.
• A sweep deletes revoked tokens at once, and expired tokens once the grace period has passed since their expiry.
• A collected token is removed entirely (secret, OAuth access token, definition and data), so it disappears from swamp access token list.
• The token state table (around line 437, the expired row) currently reads as if expired and revoked tokens stay listed. It should say they are deleted by the token GC.
• Each serve replica sweeps on its own, and deletions are pushed to a remote datastore.