docs: update for encrypt-by-default CLI

Update README.md, api.md, and error hints to reflect:
- encryption is now default (no -e flag needed)
- use -E/--no-encrypt to disable
- file path shortcut (fpaste file.txt)

app/api/routes.py

@@ -456,7 +456,7 @@ class IndexView(MethodView):
                 400,
                 size=content_size,
                 min_size=min_size,
-                hint="Encrypt content before uploading (-e flag in fpaste)",
+                hint="Encrypt content before uploading (fpaste encrypts by default)",
             )
 
         # Entropy check
@@ -476,7 +476,7 @@ class IndexView(MethodView):
                     400,
                     entropy=round(entropy, 2),
                     min_entropy=min_entropy,
-                    hint="Encrypt content before uploading (-e flag in fpaste)",
+                    hint="Encrypt content before uploading (fpaste encrypts by default)",
                 )
 
         # Binary content requirement (reject recognizable formats)
@@ -492,7 +492,7 @@ class IndexView(MethodView):
                     "Recognizable format not allowed",
                     400,
                     detected=detected_format,
-                    hint="Encrypt content before uploading (-e flag in fpaste)",
+                    hint="Encrypt content before uploading (fpaste encrypts by default)",
                 )
 
         # Deduplication check