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/input-validation.md

@@ -0,0 +1,48 @@
+# Input Validation
+
+## Purpose
+
+Protect the application.
+
+---
+
+# Validate
+
+All inputs:
+
+- Forms.
+- APIs.
+- Imports.
+- Files.
+- AI outputs.
+
+---
+
+# Protection Against
+
+Examples:
+
+- SQL injection.
+- XSS.
+- HTML injection.
+- Malicious uploads.
+
+---
+
+# CV Builder
+
+Special attention:
+
+Users can create:
+
+- Custom sections.
+- HTML-like content.
+- Formatting.
+
+The renderer must sanitise content.
+
+---
+
+# Rule
+
+Never render user content directly without validation.