docs(docs): add tech-stack badges to README (shields.io for-the-badge) (#273)

[w7-mgfcode]

Closes #273.

Summary

Adds a 31-badge block to the README between the tagline and the Features section, matching the user-provided HTML5 `for-the-badge` style. One status row + four tech-stack rows (Backend / Frontend / ML/AI / Tooling). No new files, no new assets — every badge resolves through shields.io.

Layout

• Status (4): `License: MIT` · `Release` (dynamic from GitHub releases) · `CI` (dynamic from `ci.yml` on `dev`) · `Python 3.12+`
• Backend (7): FastAPI · Pydantic v2 · SQLAlchemy 2.0 · PostgreSQL 16 · pgvector · Alembic · structlog
• Frontend (6): React 19 · TypeScript · Vite 7 · Tailwind 4 · shadcn/ui · TanStack Query
• ML / AI (7): scikit-learn · LightGBM · XGBoost · PydanticAI · OpenAI · Anthropic · Ollama
• Tooling (7): Docker · uv · pnpm · GitHub Actions · Ruff · mypy --strict · pytest

Validation

• HEAD-probe: all 31 shields URLs return HTTP 200.
• SVG logo verification: every badge that cites `&logo=` was checked for an `` element in the rendered SVG. All cited logos resolve (anthropic, astral, ruff, reactquery, ollama, shadcnui, pydantic, sqlalchemy, fastapi, postgresql, python, react, typescript, vite, tailwindcss, scikitlearn, openai, docker, pnpm, githubactions, pytest, github).
• Text-only by design (no brand icon on shields.io): LightGBM, XGBoost, structlog, mypy --strict, Alembic — render as colored text badges in matching brand colors.

Why this PR doesn't risk drift

The Release + CI badges are dynamic — they read from this repo. They self-update with every release and CI run on `dev` without any further edits to the README.

Test plan

• All 31 `https://img.shields.io/…\` URLs return 200 (HEAD probe)
• All cited `&logo=…` parameters resolve to a rendered icon in the SVG body
• Dynamic badges (Release, CI) point at this repo, not a placeholder
• Single-commit PR; ready for squash or merge

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