feat: add self-hosting support (Docker, NixOS, remote import)

[Razboy20]

Self-hosting support

Adds first-class support for self-hosting Timeful, with two deployment options documented in DEPLOYMENT.md.

Docker Compose — The primary path. Adds compose.yaml, Dockerfiles for the server and frontend, and a Caddyfile.example for the reverse proxy. The server now handles a missing .env file gracefully (env vars can be passed directly via Docker), and Cloud Tasks initialization no longer panics if unconfigured — it just disables itself with a log message.

NixOS flake — A nix/ directory contains native Nix derivations for the Go server and Vue frontend, plus a NixOS module that wires up a systemd service and a Podman-managed MongoDB instance. Consuming it is a one-liner flake input.

Import from remote instance — Adds a "Import from URL" flow (POST /api/events/import) so users can copy an event from another Timeful instance into their own. Useful when migrating or federating between self-hosted deployments.

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Infrastructure

• compose.yaml — three services: mongo (with healthcheck), frontend (build-only, copies dist to a shared volume), server (waits on both, mounts the dist read-only)
• server/Dockerfile / frontend/Dockerfile — multi-stage builds; server copies pre-built frontend dist at runtime via Docker volume
• Caddyfile.example — reverse proxy to :3002 with gzip/zstd, security headers, www redirect
• server/main.go — CORS middleware commented out (handled by reverse proxy); frontend dist path now auto-detects ./frontend/dist (Docker layout) before falling back to ../frontend/dist (local dev); missing frontend dist is a warning, not a fatal
• nix/server.nix, nix/frontend.nix — buildGoModule and buildNpmPackage derivations; client IDs passed as build args so they're baked into the frontend bundle
• nix/module.nix — NixOS module with services.timeful.{enable, domain, envFile, enableCaddy, googleClientId, microsoftClientId} options; optionally configures Caddy virtualHost; MongoDB via virtualisation.oci-containers

Frontend

• sign_in_utils.js — Google and Microsoft OAuth client IDs moved from hardcoded strings to VUE_APP_GOOGLE_CLIENT_ID / VUE_APP_MICROSOFT_CLIENT_ID env vars (set at build time via Docker build args or .env)
• TimefulImportDialog.vue — new dialog: paste a remote event URL, calls /api/events/import, emits the new event ID on success
• Home.vue — wires up the import dialog behind the existing event-creation menu

Config / server

• server/.env.template — template for required env vars
• server/routes/events.go — new POST /api/events/import endpoint: validates URL scheme, fetches the remote event and its responses via the public API, clones them locally under the authenticated user
• server/services/gcloud/tasks.go — graceful no-op when SERVICE_ACCOUNT_KEY_PATH is unset or "?"; initialization failure logs and returns instead of panicking