Add admin config screen (issue #14)

Lets the photographer edit config/site.json from /dashboard instead
of SSHing in and hand-editing the file. Two tiers:
- Quick-edit form for the fields actually touched day to day (site
  title/tagline/hero, photographer bio, contact/social, Venmo
  username, session pricing, theme colors)
- Raw JSON textarea for everything else (portfolio categories,
  locations, session types, Immich settings) - the form's fields are
  a subset of this, not a separate source of truth

Backend: GET/PUT /api/admin/config, gated by the existing requireAdmin
middleware (OIDC admin session or legacy ADMIN_SECRET). PUT validates
the body is an object with the required top-level sections, writes
atomically (temp file + rename), and keeps one prior version as
site.json.bak before overwriting.

Required a docker-compose.yml change: the api service had no volume
mount for config/ at all before this (only portfolio/nginx did, and
read-only) - added a read-write mount so the API can actually write
the file the live site reads.

docker-compose.yml

@@ -39,6 +39,10 @@ services:
       - ./api/data:/app/data
       - ./api/signed-contracts:/app/signed-contracts
       - ./api/contracts:/app/contracts:ro
+      # Read-write: the admin config screen (issue #14) edits site.json here.
+      # Shares the same host directory the portfolio/nginx service mounts
+      # read-only, so writes show up on the live site (subject to its 5-min cache).
+      - ./config:/app/config
     networks:
       - web
     healthcheck: