namespace email settings keys per adapter

Settings keys like api_key were shared across providers, so switching
from e.g. Postmark to SendGrid showed the old API key. Now each
adapter gets its own namespaced key (email_postmark_api_key, etc.)
so credentials persist independently and switching back pre-fills
previously saved values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod/mailer.ex

@@ -40,10 +40,12 @@ defmodule Berrypod.Mailer do
       adapter_info ->
         config =
           for field <- adapter_info.fields, into: %{} do
+            settings_key = Adapters.settings_key(adapter_info.key, field.key)
+
             value =
               case field.type do
-                :secret -> Settings.secret_hint("email_#{field.key}")
-                _ -> Settings.get_setting("email_#{field.key}")
+                :secret -> Settings.secret_hint(settings_key)
+                _ -> Settings.get_setting(settings_key)
               end
 
             {field.key, value}
@@ -114,7 +116,7 @@ defmodule Berrypod.Mailer do
     opts =
       for field <- adapter_info.fields, reduce: [] do
         acc ->
-          settings_key = "email_#{field.key}"
+          settings_key = Adapters.settings_key(adapter_info.key, field.key)
 
           value =
             case field.type do

lib/berrypod/mailer/adapters.ex

@@ -59,6 +59,17 @@ defmodule Berrypod.Mailer.Adapters do
         %Field{key: "secret", label: "Secret key", type: :secret, required: true}
       ]
     },
+    %Adapter{
+      key: "mailersend",
+      name: "MailerSend",
+      module: Swoosh.Adapters.MailerSend,
+      description: "Generous free tier, good analytics dashboard.",
+      tags: ["All email", "EU option"],
+      url: "https://www.mailersend.com",
+      fields: [
+        %Field{key: "api_key", label: "API key", type: :secret, required: true}
+      ]
+    },
     %Adapter{
       key: "resend",
       name: "Resend",
@@ -126,15 +137,18 @@ defmodule Berrypod.Mailer.Adapters do
     Enum.find(@adapters, &(&1.key == key))
   end
 
-  @doc "Returns the settings keys for an adapter's fields (prefixed with `email_`)."
-  def field_keys(%{fields: fields}) do
-    Enum.map(fields, &"email_#{&1.key}")
+  @doc "Returns the namespaced settings key for an adapter field."
+  def settings_key(adapter_key, field_key) do
+    "email_#{adapter_key}_#{field_key}"
+  end
+
+  @doc "Returns the settings keys for an adapter's fields."
+  def field_keys(%{key: adapter_key, fields: fields}) do
+    Enum.map(fields, &settings_key(adapter_key, &1.key))
   end
 
   @doc "Returns all possible settings keys across all adapters."
   def all_field_keys do
-    @adapters
-    |> Enum.flat_map(&field_keys/1)
-    |> Enum.uniq()
+    Enum.flat_map(@adapters, &field_keys/1)
   end
 end