feat(node): make the transfer caps settable, node-wide and per group

Step 3 of ~/next/improve-downloads.md. Step 2 built the pools with constants;
this gives them to the operator, in the two scopes they belong to.

**The pools are the machine's.** `[node] max_concurrent_downloads` and
`max_concurrent_uploads`, default 8, on the §2.11 pattern: node.toml for a
fresh install, a roster.db override for immediate effect, editable from the
Node page and from `meshbay-node transfers show|set`, applied live through the
one `set_capacity` step 1 fixed.

**The per-member cap is a group's.** How many transfers one member may run at
once here — on the node like every other group setting (not the hub, which
would have authority over someone else's disk; not node.toml, which is
hand-written and needs a restart), changed by a signed operator instruction
(`OP_TRANSFER_LIMITS`, subject "d=2,u=2" so what is signed names the outcome),
broadcast to the group, and read live by the pools.

That was the one thing step 2's shape could not express: `per_member` was a
single node-wide number. `group_limits` and `member_cap(kind, member)` make it
a lookup — the group's own value if it has one, the node's default otherwise —
and it is deliberately the only dimension that is not node-wide.

Three refusals, each with a test:

- **absent means the default (2), never "unlimited".** A group that predates
  the setting coming back unlimited would leave the node-wide pool as the only
  control, which is the situation slots exist to end;
- **zero is not "unlimited"**, and is not "this member may not transfer"
  either: the floor is one everywhere, and the CLI says to revoke the member
  instead;
- **an unreadable row reads as unset**, not as zero — the same discipline the
  sealed messages follow, where a payload that does not open must never become
  a default state on its own.

`handshake_ack` carries this member's own caps for this group, so the interface
can say "2 of your 2 slots are busy" instead of drawing a bare spinner. Absent
reads as "no limit known" and the hint is not drawn — never as "unlimited",
which would have the interface contradicting the node.

1164 node, 793 hub, 0 failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

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