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+# Quota Enforcement — Flow Diagram (rev 2.1)
+
+> **Scope** – this document explains *how* the free‑tier limits are enforced  
+> inside the scanner service.  For policy rationale and legal aspects see  
+> [`33_333_QUOTA_OVERVIEW.md`](33_333_QUOTA_OVERVIEW.md).
+
+---
+
+## 0 · Key parameters (rev 2.1)
+
+| Symbol | Value | Meaning |
+|--------|-------|---------|
+| `L_anon` | **{{ quota_anon }}** | Daily ceiling for anonymous users |
+| `L_jwt`  | **{{ quota_token }}** | Daily ceiling for token holders |
+| `T_warn` | `200` | Soft reminder threshold |
+| `D_soft` | `5 000 ms` | Delay for *first 30* over‑quota scans |
+| `D_hard` | `60 000 ms` | Delay for all scans beyond the soft window |
+
+`L_active` is `L_jwt` if a valid token is present; else `L_anon`.
+
+---
+
+## 1 · Sequence diagram
+
+```mermaid
+sequenceDiagram
+    participant C as Client
+    participant API as Scanner API
+    participant REDIS as Redis (quota)
+    C->>API: /scan
+    API->>REDIS: INCR quota:<key>
+    REDIS-->>API: new_count
+    alt new_count ≤ L_active
+        API-->>C: 202 Accepted (no delay)
+    else new_count ≤ L_active + 30
+        API->>C: wait D_soft
+        API-->>C: 202 Accepted
+    else
+        API->>C: wait D_hard
+        API-->>C: 202 Accepted
+    end
+````
+
+*Counters auto‑expire **24 h** after first increment (00:00 UTC reset).*
+
+---
+
+## 2 · Redis key layout
+
+| Key pattern            | TTL  | Description                       |
+| ---------------------- | ---- | --------------------------------- |
+| `quota:ip:<sha256>`    | 24 h | Anonymous quota per *hashed* IP   |
+| `quota:tid:<sha256>`   | 24 h | Token quota per *hashed* token‑ID |
+| `quota:ip:<sha256>:ts` | 24 h | First‑seen timestamp (ISO 8601)   |
+
+Keys share a common TTL for efficient mass expiry via `redis-cli --scan`.
+
+---
+
+## 3 · Pseudocode (Go‑style)
+
+```go
+func gate(key string, limit int) (delay time.Duration) {
+    cnt, _ := rdb.Incr(ctx, key).Result()
+
+    switch {
+    case cnt <= limit:
+        return 0             // under quota
+    case cnt <= limit+30:
+        return 5 * time.Second
+    default:
+        return 60 * time.Second
+    }
+}
+```
+
+*The middleware applies `time.Sleep(delay)` **before** processing the scan
+request; it never returns `HTTP 429` under the free tier.*
+
+---
+
+## 4 · Metrics & monitoring
+
+| Metric                         | PromQL sample                              | Alert                 |
+| ------------------------------ | ------------------------------------------ | --------------------- |
+| `stella_quota_soft_hits_total` | `increase(...[5m]) > 50`                   | Many users near limit |
+| `stella_quota_hard_hits_total` | `rate(...[1h]) > 0.1`                      | Potential abuse       |
+| Average delay per request      | `histogram_quantile(0.95, sum(rate(...)))` | P95 < 1 s expected    |
+
+---
+
+
+*Generated {{ "now" | date: "%Y‑%m‑%d" }} — values pulled from central constants.*