feat(invites): team invite links — admin create/revoke, public accept (#42)

* feat(branding): white-label — public /branding, admin write, shell theming

app_settings branding.* keys (product name, logo/favicon URLs, accent
color, docs/support links). Public GET /branding so login and shell
theme pre-auth; admin writes via PUT /settings.

useBranding() applies document.title, accent CSS var and favicon;
BrandWordmark replaces hardcoded wordmarks; admin Platform settings
gain a Branding form. MCP containr_branding_get + settings_update.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* feat(invites): team invite links — admin create/revoke, public accept

Admins mint single-use invite links (sha256-hashed token at rest,
optional email binding, configurable TTL). Public endpoints let the
accept page validate the link and register — working even while
public signup is disabled.

- migration + sqlc queries/model for user_invites
- POST /admin/invites (returns one-time token+url), GET list, DELETE revoke
- GET /auth/invites/:token public validation, POST /auth/accept-invite
- CLI containr invites list|create|revoke; MCP invites tools
- /auth/accept-invite page (expired/used/invalid states, bound-email lock)
- Admin page Team invites section: create, copy link, revoke
- OpenAPI + generated types + CLI guide

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/migrations_goose/20261105000000_user_invites.sql

@@ -0,0 +1,19 @@
+-- +goose Up
+-- Team invites: admin-generated single-use links that let a new user
+-- register even while public signup is closed. Tokens are sha256-hashed
+-- at rest like PATs.
+CREATE TABLE IF NOT EXISTS user_invites (
+    id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+    token_hash varchar(64) NOT NULL UNIQUE,
+    email varchar(255),
+    created_by uuid REFERENCES users(id) ON DELETE SET NULL,
+    expires_at timestamptz NOT NULL,
+    used_by uuid REFERENCES users(id) ON DELETE SET NULL,
+    used_at timestamptz,
+    created_at timestamptz DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_user_invites_expires ON user_invites(expires_at);
+
+-- +goose Down
+DROP TABLE IF EXISTS user_invites;