feat(relay): store per-community NIP-FI issuer bindings (#8372)

Stack: this PR → #8377 → #8399 → #8401

bb-public creates communities all the time, so a hand-edited
`BUZZ_NIP_FI_COMMUNITIES` map can't list them. This PR stores an
explicit NIP-FI issuer allowlist for each community in the database and
writes it when the community is created. Login checks don't read the
table yet; that comes in #8377 together with the database-backed
resolver. Operator steps are in
[`docs/operator-nip-fi-bindings.md`](docs/operator-nip-fi-bindings.md).

**Storage.** Migration `0066_community_nip_fi_issuers` adds
`community_nip_fi_issuers (community_id, issuer, created_at)`, with
primary key `(community_id, issuer)` and the community write fence
attached. The assertion audience is `https://<communities.host>`, so the
table has no `aud` column. Zero rows means no binding. The table is
included in whole-community deletion and purge.

**One bindable rule.** `BUZZ_NIP_FI_PROVISION_BINDING` (`{"host_suffix":
..., "authorized_issuers": [...]}`) is parsed in `buzz-auth` with
`BUZZ_NIP_FI_ISSUERS`, whatever `BUZZ_NIP_FI_MODE` is set to, and both
the relay and `buzz-admin` use the same parser. Each authorized issuer
must be a configured issuer (1 to 2048 bytes), and the suffix must not
cover any host declared in `BUZZ_NIP_FI_COMMUNITIES`; an unreadable
declaration refuses startup even in mode `off`.
`ProvisionBindingPolicy::is_bindable(host)` is true for a host exactly
one label under the suffix that is not reserved. It decides
creation-time binding, explicit `bind`, and what the lookup route
reports.

**Binding on creation.** Rows are written in the same transaction as the
community insert on all three creation paths (`POST
/operator/communities` `create_only`, its legacy convergence branch, and
startup bootstrap), and only when the call actually created the
community, so existing rows are never rewritten.

**Reserved host.** A host whose `https://<host>` equals a configured
`command_audiences` value never holds a binding. Operator provisioning
of it is rejected with a 400 and the `reserved_host` error code,
bootstrap creates it unbound, and the availability route reports it
unavailable with an error code. `main.rs` calls
`verify_reserved_hosts_unbound` at startup; if a reserved host has rows
the relay refuses to start and names the fix (`buzz-admin nip-fi unbind
--host <h> --issuer <i>`, or revert the command audience). A database
failure during that check is a separate `Check` error.

**Operator commands.**
- `buzz-admin nip-fi backfill` pages unbound communities by host (1000
per page) and binds each page with one `INSERT ... SELECT` limited to
active communities. It skips communities being deleted, always prints
`{rows_bound, unbound, unbound_hosts, truncated}`, and exits non-zero
while any bindable community is still unbound. Archived communities are
bound on purpose. Both bind paths take the shared deletion lock and then
lock the community row (`FOR NO KEY UPDATE`, in id order for a batch),
the same order deletion uses, and the backfill insert also requires the
community to still have no binding, so an operator `bind` that lands
after the backfill listed a community wins and the default issuer is not
added beside it.
- `buzz-admin nip-fi bind --host --issuer` refuses a reserved host, a
host outside the suffix, or an unconfigured issuer.
- `buzz-admin nip-fi unbind --host --issuer` refuses to remove a
community's last issuer (archive or delete the community instead), with
one exception: when a valid provisioning policy shows that backfill
can't bind the host (it's reserved, or outside the suffix), the last
issuer can be removed. That is the reserved-host recovery path in the
runbook. A missing or malformed policy doesn't block `unbind`; it just
means the exception never applies.

**Lookup route.** `GET /operator/communities/nip-fi-binding?host=`
(NIP-98, `RELAY_OPERATOR_PUBKEYS`) reads the database and returns
`{host, active, issuers[]}`. `host` is the stored host. It returns 404
for an unknown host, 200 with `active:false` for an archived or
quiescing community, and `issuers:[]` when no provisioning policy is
set. It lists only issuers that are configured and on a bindable host.
kgoose depends on this contract; it is also documented in the runbook.

**NIP-FA.** The `relay_url` bullet now also says: "An adapter MAY obtain
its exact relay list from a configured, authenticated source; it MUST
still compare `relay_url` exactly."

**Deployment dependency.** The kgoose config side is already handled by
[cash-server#137671](https://github.com/squareup/cash-server/pull/137671).
In public-community mode it requires the command audience not to be a
community audience.

---------

Signed-off-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Duncan <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@buzz.block.builderlab.xyz>
Co-authored-by: Duncan <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@buzz.block.builderlab.xyz>

schema/schema.sql

@@ -106,6 +106,7 @@ CREATE EXTENSION IF NOT EXISTS pgcrypto;
 \i tables/public/moderation_actions.sql
 \i tables/public/moderation_reports.sql
 \i tables/public/community_bans.sql
+\i tables/public/community_nip_fi_issuers.sql
 \i tables/public/_operator_global_tables.sql
 \i tables/public/git_repo_names.sql
 \i tables/public/parameterized_event_watermarks.sql
@@ -153,6 +154,7 @@ CREATE EXTENSION IF NOT EXISTS pgcrypto;
 \i triggers/audit_log.sql
 \i triggers/channel_members.sql
 \i triggers/community_bans.sql
+\i triggers/community_nip_fi_issuers.sql
 \i triggers/delivery_log.sql
 \i triggers/event_mentions.sql
 \i triggers/git_repo_names.sql

schema/tables/public/community_nip_fi_issuers.sql

@@ -0,0 +1,12 @@
+-- ── NIP-FI community issuer bindings ──────────────────────────────────────────
+-- The explicit per-community issuer allowlist for NIP-FI assertions. The
+-- assertion audience is `https://<communities.host>`, so it needs no column of
+-- its own. Zero rows means the community has no binding and NIP-FI admission
+-- for it fails closed.
+
+CREATE TABLE community_nip_fi_issuers (
+    community_id UUID NOT NULL REFERENCES communities(id),
+    issuer       TEXT NOT NULL CHECK (length(issuer) BETWEEN 1 AND 2048),
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (community_id, issuer)
+);

schema/triggers/community_nip_fi_issuers.sql

@@ -0,0 +1 @@
+SELECT attach_community_write_fence('community_nip_fi_issuers');