Add third-party self-referential relations

Introduce a self-referential many-to-many relation table so a
third party can have child third parties. Each relation is
directional (parent to child); both directions can coexist as
independent rows.

Add a first_level boolean on third_parties (default true) with
a filter on the list page that defaults to showing only
first-level third parties.

Frontend adds a "Third Parties" tab on the detail page where
users can link existing third parties or create new ones from
the common third party catalog (created as non-first-level).
The list page gets a First Level/All toggle filter.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/migrations/20260519T100000Z.sql

@@ -0,0 +1,25 @@
+-- Copyright (c) 2025-2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE third_parties ADD COLUMN first_level boolean NOT NULL DEFAULT true;
+ALTER TABLE third_parties ALTER COLUMN first_level DROP DEFAULT;
+
+CREATE TABLE third_party_third_parties (
+    parent_third_party_id text NOT NULL REFERENCES third_parties(id) ON DELETE CASCADE,
+    child_third_party_id  text NOT NULL REFERENCES third_parties(id) ON DELETE CASCADE,
+    tenant_id             bytea NOT NULL,
+    created_at            timestamptz NOT NULL,
+    PRIMARY KEY (parent_third_party_id, child_third_party_id),
+    CHECK (parent_third_party_id <> child_third_party_id)
+);