ML-154: add early platform detection for SQLite extensions

lib/music_library/repo.ex

@@ -1,25 +1,41 @@
 defmodule MusicLibrary.Repo do
+  @moduledoc """
+  Main application repository using SQLite3 with FTS5 unicode and vec0 extensions.
+  """
+
   use Ecto.Repo,
     otp_app: :music_library,
     adapter: Ecto.Adapters.SQLite3
 
+  @doc """
+  Returns `:ok` if the current OS/architecture has precompiled SQLite extensions
+  available (unicode, vec0). Raises `RuntimeError` with a diagnostic message
+  including the detected platform and supported alternatives.
+  """
+  def ensure_supported_platform! do
+    case platform_string() do
+      {:ok, _platform} -> :ok
+      {:error, os, arch} -> raise_unsupported_platform(os, arch)
+    end
+  end
+
+  @doc """
+  Returns `true` if the current OS/architecture has precompiled SQLite extensions
+  available (unicode, vec0).
+  """
+  def supported_platform? do
+    match?({:ok, _}, platform_string())
+  end
+
+  @doc """
+  Returns the filesystem path for a named SQLite extension.
+
+  Only macOS (Intel/Apple Silicon) and Linux (amd64/arm64) are supported.
+  """
   def extension_path(name) do
-    [arch | _rest] =
-      :erlang.system_info(:system_architecture)
-      |> List.to_string()
-      |> String.split("-")
-
+    platform = platform_string!()
     {_, os} = :os.type()
 
-    platform =
-      case {os, arch} do
-        {:darwin, "x86_64"} -> "darwin-amd64"
-        {:darwin, "aarch64"} -> "darwin-arm64"
-        {:linux, "x86_64"} -> "linux-amd64"
-        {:linux, "aarch64"} -> "linux-arm64"
-        _other -> raise "Unsupported OS or platform"
-      end
-
     extension =
       case os do
         :darwin -> "dylib"
@@ -34,6 +50,49 @@ defmodule MusicLibrary.Repo do
     ])
   end
 
+  defp raise_unsupported_platform(os, arch) do
+    raise """
+    Unsupported platform: #{os} / #{arch}
+
+    This application ships precompiled SQLite extensions (unicode, vec0) for:
+      • darwin-amd64  (macOS Intel)
+      • darwin-arm64  (macOS Apple Silicon)
+      • linux-amd64   (Linux x86_64)
+      • linux-arm64   (Linux aarch64)
+
+    Your platform (#{inspect(os)} / #{arch}) is not supported. The SQLite extensions
+    required for full-text search (unicode) and vector similarity search (vec0)
+    cannot be loaded on this architecture.
+
+    To run on this platform, provide compatible builds of these extensions in
+    priv/sqlite_extensions/#{os}-#{arch}/.
+    """
+  end
+
+  defp platform_string! do
+    case platform_string() do
+      {:ok, platform} -> platform
+      {:error, _os, _arch} -> raise "Unsupported OS or platform"
+    end
+  end
+
+  defp platform_string do
+    [arch | _rest] =
+      :erlang.system_info(:system_architecture)
+      |> List.to_string()
+      |> String.split("-")
+
+    {_, os} = :os.type()
+
+    case {os, arch} do
+      {:darwin, "x86_64"} -> {:ok, "darwin-amd64"}
+      {:darwin, "aarch64"} -> {:ok, "darwin-arm64"}
+      {:linux, "x86_64"} -> {:ok, "linux-amd64"}
+      {:linux, "aarch64"} -> {:ok, "linux-arm64"}
+      _ -> {:error, os, arch}
+    end
+  end
+
   def vacuum, do: query("VACUUM")
 
   def optimize, do: query("PRAGMA optimize")