Add documentation pages with interactive architecture diagram

- /docs route with sidebar navigation and index page
- Architecture section: system overview with React Flow diagram, data flow, database schema
- API reference, configuration guide, and self-hosting docs for Go DNS server
- Feature planning document for future development
- Added docs link to landing page nav

docs/feature-plan.md

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+# VectorDNS Feature Plan
+
+Decisions captured for current and future development.
+
+---
+
+## Auth & User System
+
+| Feature | Decision | Priority |
+|---|---|---|
+| OAuth providers | GitHub + Google | MVP |
+| Email/password + magic link | Yes | MVP |
+| Team/org accounts | Yes, eventually — shared watchlists, role-based access (admin/viewer) | Future |
+| User API keys | No — Go server is internal, called only by the Next.js backend | N/A |
+
+### Schema implications (teams)
+- Future `teams` table with membership + roles
+- `saved_domains` will need an optional `team_id` foreign key
+- RLS policies will need team-aware variants
+
+---
+
+## DNS & Monitoring
+
+| Feature | Decision | Priority |
+|---|---|---|
+| Check intervals | Daily (MVP), then hourly/daily/weekly selectable | MVP → Near-term |
+| Realtime monitoring | Not now — possible distant future | Deferred |
+| Custom resolvers | Fixed resolvers (Google 8.8.8.8, Cloudflare 1.1.1.1) by default; user-specified resolvers as future feature | MVP → Future |
+| Propagation checks | Query multiple resolvers and compare results | Near-term |
+| DNSSEC validation | Basic AD flag check (current), deep chain validation later | MVP → Future |
+
+### Go server implications
+- Scheduler needed for flexible intervals (hourly/daily/weekly per domain)
+- Resolver list should be configurable, not hardcoded (prep for custom resolvers)
+- Propagation endpoint: query N resolvers in parallel, return per-resolver results
+
+---
+
+## Alert Channels
+
+| Channel | Priority |
+|---|---|
+| In-app notification feed | MVP |
+| Email (Resend) | MVP |
+| Webhooks (user-configured URL) | Near-term |
+| Slack integration | Future |
+| Discord integration | Future |
+
+### Schema implications
+- `notification_channels` table: user_id, type (email/webhook/slack/discord), config (jsonb), enabled
+- Webhook: store URL + optional secret for HMAC signing
+- Slack/Discord: store webhook URL or OAuth token
+
+---
+
+## Dashboard & UX
+
+| Feature | Decision | Priority |
+|---|---|---|
+| Domain organization | Tags (current) + folders/groups | Near-term |
+| Public sharing | Shareable links for domain DNS snapshots | Near-term |
+| Data export | CSV + JSON for DNS history, saved domains, notifications | Near-term |
+| Bulk operations | Add/remove/re-check multiple domains at once | Future |
+| Search & filter | Filter by tag, folder, record type, change status | Near-term |
+
+### Schema implications (folders)
+- `domain_folders` table: id, user_id, name, created_at
+- `saved_domains` gets an optional `folder_id` foreign key
+- RLS policies for folder ownership
+
+### Schema implications (public sharing)
+- `shared_snapshots` table: id, saved_domain_id, share_token (unique), dns_snapshot (jsonb), expires_at, created_at
+- Public access via token — no auth required to view
+
+---
+
+## Infrastructure
+
+| Feature | Decision | Priority |
+|---|---|---|
+| Billing/paid tiers | Freemium — free tier with limits, paid for more domains/features/intervals | Future |
+| DNS caching | Redis — external cache for TTL-based DNS result caching | Near-term |
+| Self-hosting | Go server only — provide Dockerfile + docker-compose for the DNS API | Near-term |
+| Next.js hosting | Vercel only, not self-hostable | — |
+
+### Freemium tier structure (draft)
+- **Free**: 5 monitored domains, daily checks, in-app + email alerts
+- **Pro**: 50 domains, hourly checks, webhooks, folders, export
+- **Team**: everything in Pro + team accounts, shared watchlists
+
+### Go server additions needed
+- Redis client (go-redis) for caching layer
+- Cache middleware: check Redis before querying DNS, store with TTL
+- docker-compose.yml with Go server + Redis for self-hosting
+- Rate limiting per API key / tier (when billing is added)
+
+---
+
+## Summary of future schema additions
+
+These tables will be needed as features are built:
+
+| Table | For |
+|---|---|
+| `teams` | Team/org accounts |
+| `team_members` | Membership + roles |
+| `domain_folders` | Folder organization |
+| `shared_snapshots` | Public shareable links |
+| `notification_channels` | Webhook/Slack/Discord config |
+| `subscriptions` | Billing tier tracking |