feat(07.3-02): ES256 JWT algorithm upgrade + startup rotation hook

- config.py: add refresh_token_expire_hours=16, jwt_private_key, jwt_public_key fields (D-01, D-09)
- services/auth.py: swap all 4 JWT sites to ES256 via base64-decoded PEM keys; remove HS256 (D-02, D-03)
- main.py: add _rotate_tokens_on_algorithm_change lifespan hook — bulk-revokes refresh tokens on algorithm change; idempotent on repeat boots (D-04, D-05)
- test_auth_es256.py: promote ES256-01..05 + CFG-01 stubs to 6 passing tests; RM-01..03 remain xfail
- docker-compose.yml: inject JWT_PRIVATE_KEY + JWT_PUBLIC_KEY into backend + celery-worker (D-07)
- README.md: add JWT key env vars + key generation Python one-liner snippet
- .env.example: add JWT_PRIVATE_KEY= and JWT_PUBLIC_KEY= lines
- Version bump to 0.1.2

.env.example

@@ -31,6 +31,11 @@ REDIS_URL=redis://:changeme_redis@redis:6379/0
 # JWT signing secret — generate with: python3 -c "import secrets; print(secrets.token_hex(64))"
 SECRET_KEY=CHANGEME-replace-with-64-char-random-hex
 
+# ── ES256 JWT Keypair (Phase 7.3) ─────────────────────────────────────────────
+# Generated by running the Python snippet in README.md JWT Key Generation section.
+JWT_PRIVATE_KEY=
+JWT_PUBLIC_KEY=
+
 # ── Admin Bootstrap (Phase 2 — D-04) ─────────────────────────────────────────
 # First admin account created on startup if users table is empty.
 # Both vars must be set; if missing, a WARNING is logged but app starts normally.