feat(aether): initial commit – self-hosted Cloud, Phasen 1–7

Go-Backend (chi, pgx, golang-migrate) + React/TS-Frontend als PWA.
Enthalten: Auth inkl. OIDC, Dateien mit Versionen und Papierkorb,
Freigaben (Links + intern), WebDAV, Gruppenordner, Admin/Theming,
Volltextsuche mit OCR, eigener PDF-Betrachter und WOPI-Office
(Collabora/OnlyOffice).

Nicht im Repo: deploy/.env (Sitzungsschlüssel, DB- und Admin-Passwort)
sowie alle Daten- und Build-Verzeichnisse – siehe .gitignore.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/db/migrations/000004_phase3_admin.up.sql

@@ -0,0 +1,35 @@
+-- Phase 3: Admin panel – groups, settings, user display names
+
+-- ── Users: display name ───────────────────────────────────────────────────────
+ALTER TABLE users ADD COLUMN IF NOT EXISTS display_name TEXT NOT NULL DEFAULT '';
+
+-- ── Groups ───────────────────────────────────────────────────────────────────
+CREATE TABLE groups (
+    id          UUID        PRIMARY KEY DEFAULT gen_random_uuid(),
+    name        TEXT        NOT NULL UNIQUE,
+    description TEXT        NOT NULL DEFAULT '',
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE group_members (
+    group_id    UUID        NOT NULL REFERENCES groups(id)  ON DELETE CASCADE,
+    user_id     UUID        NOT NULL REFERENCES users(id)   ON DELETE CASCADE,
+    joined_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (group_id, user_id)
+);
+
+CREATE INDEX idx_group_members_user ON group_members(user_id);
+
+-- ── Settings (key-value, admin-managed) ───────────────────────────────────────
+CREATE TABLE settings (
+    key         TEXT        PRIMARY KEY,
+    value       TEXT        NOT NULL DEFAULT '',
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Default settings
+INSERT INTO settings (key, value) VALUES
+    ('cloud_name',     'Aether'),
+    ('primary_color',  '#6366f1'),
+    ('accent_color',   '#8b5cf6'),
+    ('logo_storage_key', '');