feat: ad engagement, invite gate, sidebar footer, docker setup

Ad interactions (reactions/comments/reposts on ads):
- Schema: AdReaction, AdComment, AdRepost models with RLS enabled
- tRPC: new ads router (getFeedData, toggleReaction, addComment, listComments, toggleRepost)
- AdCard: fully interactive card with paw, comment sheet, repost — mirrors PostCard style
- getActiveAds includes _count for advertiser engagement metrics

Invite system enforcement:
- proxy.ts: /sign-up blocked without pf_invite cookie
- INVITE_REQUIRED=false env var disables gate for post-beta live launch
- Full invite flow: /join → cookie → /sign-up → consume-invite → 3 codes issued to new user
- Admin: listInviteCodes, createRootInvite, revokeInvite procedures

Sidebar mini-footer:
- Impressum + Datenschutz links always visible at sidebar bottom
- PawFeed Alpha-Test copyright line

Docker self-hosting:
- docker/Dockerfile: 3-stage build using Next.js standalone output
- docker/docker-compose.yml: env_file + build-args for NEXT_PUBLIC_ vars
- .dockerignore at project root
- next.config.ts: output standalone for minimal image
- .env.example: documented INVITE_REQUIRED

prisma/enable_rls_invitecode.sql

@@ -0,0 +1 @@
+ALTER TABLE "InviteCode" ENABLE ROW LEVEL SECURITY;