entropy: exempt small content from check

Small data has unreliable entropy measurement due to sample size.
MIN_ENTROPY_SIZE (default 256 bytes) sets the threshold.

documentation/api.md

@@ -355,7 +355,8 @@ FlaskPaste can require minimum content entropy to enforce client-side encryption
 export FLASKPASTE_MIN_ENTROPY=6.0      # Require encryption-level entropy (0=disabled)
 export FLASKPASTE_MIN_ENTROPY_SIZE=256 # Only check content >= this size (default: 256)
 ```
-**Response (400 Bad Request):**
+
+**Response (400 Bad Request):**
 ```json
 {
   "error": "Content entropy too low",
@@ -369,7 +370,7 @@ export FLASKPASTE_MIN_ENTROPY=7.0  # Require ~encryption-level entropy (0=disabl
 - Small data is exempt (configurable via `MIN_ENTROPY_SIZE`, default 256 bytes)
 - Compressed data (gzip, zip) also has high entropy — not distinguishable from encrypted
 - This is a heuristic, not cryptographic proof of encryption
-
+
 **Recommended thresholds:**
 | Threshold | Effect |
 |-----------|--------|