docs: reorganize tree, restore architecture + research from archive, add Phase 0 reports

Active docs/ was stub scaffolding while the real docs sat in docs/_archive/.
Restore and correct them, and record the Phase 0 work.

- docs/architecture/current.md: verified system map (from archived SYSTEM_OVERVIEW,
  9 corrections against code).
- docs/research/competitors.md: sourced competitor analysis (from archived
  PRODUCT_RESEARCH, feature matrix corrected).
- docs/decisions/ADR-002-job-application-model.md: the Job/JobApplication split.
- docs/application-discovery-report.md, docs/implementation-roadmap.md,
  docs/phase-0-foundation-report.md, docs/career-workspace-branch-assessment.md.
- Remove 10 zero-byte placeholder files that advertised content that never existed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/security/saas-security.md

@@ -0,0 +1,51 @@
+# SaaS Security
+
+## Future Requirements
+
+When moving to SaaS:
+
+Support:
+
+- Multiple users.
+- Subscription plans.
+- Usage limits.
+
+---
+
+# Tenant Isolation
+
+Every resource belongs to:
+
+User
+
+or
+
+Organisation.
+
+---
+
+# Billing Security
+
+Never trust client-side:
+
+- Subscription status.
+- Permissions.
+- Limits.
+
+Verify server-side.
+
+---
+
+# Admin Features
+
+Protect:
+
+- User management.
+- AI providers.
+- System settings.
+
+---
+
+# Principle
+
+Prepare architecture without adding unnecessary complexity early.