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/architecture/backend.md

@@ -0,0 +1,75 @@
+# Backend Architecture
+
+## Purpose
+
+Define backend organisation.
+
+---
+
+# Preferred Structure
+
+Example:
+
+Application
+
+├── Features
+
+├── Domain
+
+├── Infrastructure
+
+├── Services
+
+├── API
+
+---
+
+# Responsibilities
+
+## API Layer
+
+Handles:
+
+- HTTP requests.
+- Authentication.
+- Validation.
+- Responses.
+
+Should not contain business logic.
+
+---
+
+## Application Layer
+
+Handles:
+
+- Workflows.
+- Business operations.
+- Coordination.
+
+---
+
+## Domain Layer
+
+Contains:
+
+- Entities.
+- Rules.
+- Business concepts.
+
+---
+
+## Infrastructure
+
+Handles:
+
+- Database.
+- External APIs.
+- File storage.
+- AI providers.
+
+---
+
+# Rule
+
+Controllers should remain thin.