docs: add centralized documentation hub

[maxachis]

Summary

• Creates a docs/ directory with 10 documentation files covering architecture, API (overview, authentication, endpoints), development (setup, database, testing, workflow), and troubleshooting.
• Slims down README.md to a quick-start guide with links to the new docs.
• All content written from the actual codebase — covers the dual Flask/FastAPI architecture, all 29 v2 namespaces + 3 v3 routers, auth model, database schema, CI pipeline, and common issues.

New files

docs/
├── README.md                      # Index linking to all docs
├── architecture.md                # System design, request flow, directory structure
├── api/
│   ├── overview.md                # v2 vs v3, auto-docs, migration plan
│   ├── authentication.md          # JWT, API keys, OAuth, permissions
│   └── endpoints.md               # All endpoint namespaces
├── development/
│   ├── setup.md                   # Full setup (uv + pip, Docker DB, sandbox)
│   ├── database.md                # Schema, query builders, Alembic
│   ├── testing.md                 # Running/writing tests, CI
│   └── workflow.md                # Branching, PRs, CI checks, code standards
└── troubleshooting.md             # Common issues and fixes

Test plan

• Verify all markdown renders correctly on GitHub
• Verify all internal links between docs resolve
• Verify links back to existing in-repo READMEs work
• Review content accuracy against current codebase

🤖 Generated with Claude Code