refactor(tenant): drop legacy plugin_installations, migrate monitoring to desktop_clients

Hard cutover from the browser-extension plugin flow to desktop clients:
remove plugin_installations/plugin_sessions tables and related service,
handler, router, and generated model code; migrate monitoring quotas
and collector types to desktop_clients (UUID primary_client_id);
recreate platform_access_snapshots keyed by client_id; update dev-seed
and callback types accordingly; mark legacy design docs as historical.

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

server/migrations/20260402153000_repair_plugin_installations_schema.up.sql

@@ -1,39 +1 @@
-CREATE TABLE IF NOT EXISTS plugin_installations (
-    id                      BIGSERIAL PRIMARY KEY,
-    tenant_id               BIGINT NOT NULL REFERENCES tenants(id),
-    user_id                 BIGINT NOT NULL REFERENCES users(id),
-    installation_key        VARCHAR(128) NOT NULL,
-    installation_name       VARCHAR(255) NOT NULL,
-    browser_name            VARCHAR(64),
-    client_version          VARCHAR(32),
-    installation_token_hash VARCHAR(128) NOT NULL,
-    status                  VARCHAR(20) NOT NULL DEFAULT 'active',
-    last_seen_at            TIMESTAMPTZ,
-    created_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-    updated_at              TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-    deleted_at              TIMESTAMPTZ
-);
-
-CREATE UNIQUE INDEX IF NOT EXISTS uk_plugin_installations_key_active
-    ON plugin_installations(tenant_id, installation_key)
-    WHERE deleted_at IS NULL;
-
-CREATE INDEX IF NOT EXISTS idx_plugin_installations_tenant_status
-    ON plugin_installations(tenant_id, status, created_at DESC);
-
-ALTER TABLE plugin_sessions
-    ADD COLUMN IF NOT EXISTS plugin_installation_id BIGINT;
-
-DO $$
-BEGIN
-    IF NOT EXISTS (
-        SELECT 1
-        FROM pg_constraint
-        WHERE conname = 'plugin_sessions_plugin_installation_id_fkey'
-    ) THEN
-        ALTER TABLE plugin_sessions
-            ADD CONSTRAINT plugin_sessions_plugin_installation_id_fkey
-            FOREIGN KEY (plugin_installation_id)
-            REFERENCES plugin_installations(id);
-    END IF;
-END $$;
+-- Legacy no-op.