feat(sp19): security hardening + rich health probe

Three pure-ASGI middlewares close completeness-review gaps §3.1.4
(no body/rate limits) and §3.1.25 (no security headers):

- BodySizeLimitMiddleware — rejects oversized uploads (50 MB
  default, CYCLONE_MAX_BODY_BYTES override). 413 on over-cap
  Content-Length and on chunked reads that cross the cap.
- RateLimitMiddleware — sliding-window per-IP limiter (300/min
  default, CYCLONE_RATE_LIMIT_PER_MIN override). 429 over the
  window. /api/health is exempt.
- SecurityHeadersMiddleware — stamps X-Content-Type-Options,
  X-Frame-Options, Referrer-Policy, Permissions-Policy, and a
  strict Content-Security-Policy on every response.

Every 413/429 also writes a tamper-evident api.request_rejected
event into the SP11 audit chain so an operator can correlate
rejections with the SP18 JSON logs.

GET /api/health is rewritten to return a subsystem snapshot:
DB connectivity (SELECT 1), MFT scheduler state, backup scheduler
state, live pubsub subscriber counts, last batch id + timestamp.
Returns status='degraded' if any subsystem is unhappy; per-subsystem
errors surfaced in the respective dict.

Cyclone.pubsub.EventBus.stats() — new method for live subscriber
counts.

13 new tests (test_security.py) + 1 updated (test_api.py health
endpoint). All 883 backend tests pass.

README.md

@@ -370,6 +370,60 @@ fingerprints and the post-rotation table count. The old key is
 retained in the `cyclone.db.key.previous` Keychain account for a
 grace period so a botched rotation can be rolled back by hand.
 
+## Security hardening (SP19)
+
+Three pure-ASGI middlewares sit in front of every FastAPI request.
+They're sized for Cyclone's local-only posture — a misconfigured
+Tailscale / ngrok bind, a buggy cron job uploading a 4 GB file, or a
+port-scraper — not for hostile internet exposure.
+
+| Middleware | Default | Override | Reject |
+|------------|---------|----------|--------|
+| `BodySizeLimitMiddleware` | 50 MB | `CYCLONE_MAX_BODY_BYTES` | `413 body_too_large` over Content-Length cap; chunked reads capped too |
+| `RateLimitMiddleware` | 300 req/min/IP | `CYCLONE_RATE_LIMIT_PER_MIN` | `429 rate_limited` over the sliding window; `/api/health` exempt |
+| `SecurityHeadersMiddleware` | always on | n/a | stamps `X-Content-Type-Options: nosniff`, `X-Frame-Options: DENY`, `Referrer-Policy: same-origin`, `Permissions-Policy`, `Content-Security-Policy: default-src 'none'; frame-ancestors 'none'` |
+
+Every rejection (413 / 429) also writes a tamper-evident
+`api.request_rejected` event into the SP11 audit chain so an
+operator can correlate a misbehaving client with the SP18 JSON logs:
+
+```json
+{"event_type":"api.request_rejected","entity_id":"POST /api/parse-837","payload":{"status":413,"reason":"body_too_large","path":"/api/parse-837","method":"POST","ip":"127.0.0.1"}}
+```
+
+### Health probe
+
+`GET /api/health` now returns a subsystem snapshot:
+
+```json
+{
+  "status": "ok",
+  "version": "0.1.0",
+  "db":      {"ok": true},
+  "scheduler": {"running": true, "interval_s": 60, "sftp_block": "co_medicaid",
+                "backup_scheduler_running": false, "backup_interval_hours": 24.0},
+  "pubsub":  {"parse_completed": 1, "batch_added": 1},
+  "batch":   {"last_batch_id": 42, "last_batch_kind": "837P",
+              "last_batch_at": "2026-06-21T15:30:00.123Z",
+              "last_batch_filename": "TP11525703-837P-..."}
+}
+```
+
+Returns `"status": "degraded"` if any subsystem reports an error —
+the per-subsystem dict still surfaces so an operator can see which
+one is unhappy. `/api/health` is rate-limit exempt so a load balancer
+hammering the endpoint doesn't trip the limiter.
+
+### Files
+
+* `cyclone.security` — `BodySizeLimitMiddleware`,
+  `RateLimitMiddleware`, `SecurityHeadersMiddleware`, and
+  `get_health_snapshot()` (~330 LOC).
+* `cyclone.api_routers.health` rewritten to use `get_health_snapshot()`.
+* `cyclone.pubsub.EventBus.stats()` — new method that returns
+  per-kind subscriber counts.
+* `tests/test_security.py` — 13 new tests.
+
 ## Structured logging (SP18)
 
 Cyclone emits newline-delimited JSON to stderr by default — readable
@@ -599,7 +653,7 @@ backup API).
 
 ## Roadmap
 
-Sub-projects 2 through 18 are **shipped**. See the [completeness
+Sub-projects 2 through 19 are **shipped**. See the [completeness
 review](docs/reviews/2026-06-20-cyclone-completeness-review.md) for
 the honest gap analysis against the industry definition of a HIPAA
 clearinghouse — the short version is that the local-only,
@@ -610,6 +664,16 @@ scope.
 
 Shipped sub-projects (most recent first):
 
+- **Sub-project 19 (shipped) — Security hardening + health probe.**
+  Three pure-ASGI middlewares (`BodySizeLimitMiddleware`,
+  `RateLimitMiddleware`, `SecurityHeadersMiddleware`) close the
+  completeness-review gaps §3.1.4 (no body/rate limits) and §3.1.25
+  (no CSP / security headers). 413/429 rejections emit a
+  tamper-evident `api.request_rejected` audit event (SP11 chain).
+  `/api/health` is now a rich subsystem snapshot — DB connectivity,
+  MFT scheduler state, backup scheduler state, live pubsub
+  subscriber counts, last batch id + timestamp. See
+  [Security hardening (SP19)](#security-hardening-sp19) below.
 - **Sub-project 18 (shipped) — Structured JSON logging.** All logs
   emitted by the API, CLI, scheduler tick loop, and backup service
   flow through a `JsonFormatter` (newline-delimited JSON, ISO-8601 ms