feat(backends): Pool-Modell — Backend = Pool, N Server pro Backend

Migration 0016: backend_servers (id, backend_id, name, address, port,
weight, backup, active) + backends.lb_algorithm. Daten-Migration kopiert
bestehende backends.address/port als ersten Server, dann DROP COLUMN.

HAProxy-Renderer: rendert pro Backend einen Block mit `balance <algo>`
+ N `server`-Zeilen (weight, backup-Flag, optional check inter 5s).
LB-Algorithmen: roundrobin / leastconn / source.

REST: /backends/:id/servers (GET/POST), /backend-servers/:id (PUT/DELETE).
Re-rendert HAProxy nach jeder Server-Mutation.

UI: address/port aus Backend-Form raus, lb_algorithm-Select rein. Server
verwaltet ein expandable Sub-Panel pro Backend-Row (Tabelle + Add/Edit/
Delete-Modal). Domain-Attachment-Multi-Select bleibt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/database/migrations/0016_backend_servers.sql

@@ -0,0 +1,81 @@
+-- +goose Up
+-- +goose StatementBegin
+
+-- v1.0.49: backends wird vom Single-Server-Endpoint zum Pool.
+-- Bisher: backends.address/port → genau ein Upstream.
+-- Jetzt: backends = Pool-Definition (LB-Algo, Healthcheck), die
+-- konkreten Upstreams leben in backend_servers (1:N).
+-- Damit kann eine Domain → ein Backend → N Server (HAProxy macht
+-- den Loadbalance laut backends.lb_algorithm).
+
+CREATE TABLE IF NOT EXISTS backend_servers (
+    id          BIGSERIAL PRIMARY KEY,
+    backend_id  BIGINT NOT NULL REFERENCES backends(id) ON DELETE CASCADE,
+    name        TEXT NOT NULL,
+    address     TEXT NOT NULL,
+    port        INTEGER NOT NULL,
+    weight      INTEGER NOT NULL DEFAULT 100,
+    backup      BOOLEAN NOT NULL DEFAULT FALSE,
+    active      BOOLEAN NOT NULL DEFAULT TRUE,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    CONSTRAINT backend_servers_name_unique  UNIQUE (backend_id, name),
+    CONSTRAINT backend_servers_port_check   CHECK (port > 0 AND port < 65536),
+    CONSTRAINT backend_servers_weight_check CHECK (weight >= 0 AND weight <= 256)
+);
+
+CREATE INDEX IF NOT EXISTS idx_backend_servers_backend
+    ON backend_servers (backend_id);
+CREATE INDEX IF NOT EXISTS idx_backend_servers_active
+    ON backend_servers (backend_id, active) WHERE active;
+
+-- LB-Algorithmus pro Backend. Default roundrobin entspricht haproxy-
+-- default; source = sticky-per-Source-IP (für stateful Apps ohne
+-- shared session store).
+ALTER TABLE backends
+    ADD COLUMN IF NOT EXISTS lb_algorithm TEXT NOT NULL DEFAULT 'roundrobin';
+
+ALTER TABLE backends
+    DROP CONSTRAINT IF EXISTS backends_lb_check;
+ALTER TABLE backends
+    ADD CONSTRAINT backends_lb_check
+        CHECK (lb_algorithm IN ('roundrobin', 'leastconn', 'source'));
+
+-- Daten-Migration: jede bisherige backends-Row produziert genau eine
+-- backend_servers-Row mit demselben address/port. name = backends.name
+-- damit der HAProxy-server-id stabil bleibt (haproxy stats / logs
+-- referenzieren ihn). Idempotent — bei Re-Apply gibt's einen Skip
+-- über das Unique-Constraint.
+INSERT INTO backend_servers (backend_id, name, address, port, active)
+SELECT id, name, address, port, active
+  FROM backends
+ WHERE address IS NOT NULL AND port IS NOT NULL
+ON CONFLICT (backend_id, name) DO NOTHING;
+
+-- Erst nach erfolgreichem Insert die alten Spalten droppen. Wenn die
+-- Migration mittendrin abbricht, sind die Daten in beiden Tabellen
+-- vorhanden — wir verlieren nichts.
+ALTER TABLE backends DROP COLUMN IF EXISTS address;
+ALTER TABLE backends DROP COLUMN IF EXISTS port;
+
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+ALTER TABLE backends ADD COLUMN IF NOT EXISTS address TEXT;
+ALTER TABLE backends ADD COLUMN IF NOT EXISTS port    INTEGER;
+
+UPDATE backends b
+   SET address = bs.address,
+       port    = bs.port
+  FROM (
+    SELECT DISTINCT ON (backend_id) backend_id, address, port
+      FROM backend_servers
+     ORDER BY backend_id, id ASC
+  ) bs
+ WHERE b.id = bs.backend_id;
+
+ALTER TABLE backends DROP CONSTRAINT IF EXISTS backends_lb_check;
+ALTER TABLE backends DROP COLUMN IF EXISTS lb_algorithm;
+DROP TABLE IF EXISTS backend_servers;
+-- +goose StatementEnd