fix: A1-7 implement 4-level template lookup cascade (post→tag→category→default)

lib/bds/generation/outputs.ex

@@ -5,6 +5,8 @@ defmodule BDS.Generation.Outputs do
   import BDS.Generation.Renderers
   import BDS.Generation.Sitemap, only: [render_feed: 3, render_atom: 3, render_calendar: 1]
 
+  alias BDS.Rendering.TemplateSelection
+
   @spec additional_languages(map()) :: [String.t()]
   def additional_languages(plan) do
     Enum.reject(plan.blog_languages, &(&1 == plan.language))
@@ -391,10 +393,12 @@ defmodule BDS.Generation.Outputs do
         canonical_variant = Map.get(translations_by_post_language, {post.id, main_language}, post)
         body = load_body(project_id, canonical_variant.file_path, canonical_variant.content)
 
+        effective_slug = effective_template_slug(project_id, post)
+
         {page_output_path(post.slug, nil),
          render_post_output(
            project_id,
-           post.template_slug,
+           effective_slug,
            %{
              id: canonical_variant.id,
              title: canonical_variant.title,
@@ -423,10 +427,12 @@ defmodule BDS.Generation.Outputs do
         |> Enum.map(fn post ->
           body = load_body(project_id, post.file_path, post.content)
 
+          effective_slug = effective_template_slug(project_id, post)
+
           {page_output_path(post.slug, language),
            render_post_output(
              project_id,
-             post.template_slug,
+             effective_slug,
              %{
                id: post.id,
                title: post.title,
@@ -521,10 +527,12 @@ defmodule BDS.Generation.Outputs do
         canonical_variant = Map.get(translations_by_post_language, {post.id, main_language}, post)
         body = load_body(project_id, canonical_variant.file_path, canonical_variant.content)
 
+        effective_slug = effective_template_slug(project_id, post)
+
         {post_output_path(post),
          render_post_output(
            project_id,
-           post.template_slug,
+           effective_slug,
            %{
              id: canonical_variant.id,
              title: canonical_variant.title,
@@ -551,10 +559,12 @@ defmodule BDS.Generation.Outputs do
         Enum.map(posts, fn post ->
           body = load_body(project_id, post.file_path, post.content)
 
+          effective_slug = effective_template_slug(project_id, post)
+
           {post_output_path(post, language),
            render_post_output(
              project_id,
-             post.template_slug,
+             effective_slug,
              %{
                id: post.id,
                title: post.title,
@@ -571,4 +581,18 @@ defmodule BDS.Generation.Outputs do
 
     post_outputs ++ translation_outputs
   end
+
+  defp effective_template_slug(project_id, post) do
+    slug = Map.get(post, :template_slug)
+
+    if is_binary(slug) and slug != "" do
+      slug
+    else
+      TemplateSelection.resolve_post_template_slug(
+        project_id,
+        Map.get(post, :tags) || [],
+        Map.get(post, :categories) || []
+      )
+    end
+  end
 end

lib/bds/preview.ex

@@ -9,6 +9,7 @@ defmodule BDS.Preview do
   alias BDS.Projects
   alias BDS.Repo
   alias BDS.Rendering
+  alias BDS.Rendering.TemplateSelection
 
   @host "127.0.0.1"
   @port 4123
@@ -219,6 +220,7 @@ defmodule BDS.Preview do
 
   defp draft_preview_payload(post, query_params) do
     requested_language = query_params |> Map.get("lang") |> normalize_requested_language()
+    effective_slug = post.template_slug || TemplateSelection.resolve_post_template_slug(post.project_id, post.tags, post.categories)
 
     case draft_preview_translation(post.id, requested_language, post.language) do
       %Translation{} = translation ->
@@ -230,7 +232,7 @@ defmodule BDS.Preview do
           slug: post.slug,
           language: translation.language,
           excerpt: translation.excerpt,
-          template_slug: post.template_slug
+          template_slug: effective_slug
         }
 
       nil ->
@@ -242,7 +244,7 @@ defmodule BDS.Preview do
           slug: post.slug,
           language: post.language,
           excerpt: post.excerpt,
-          template_slug: post.template_slug
+          template_slug: effective_slug
         }
     end
   end

lib/bds/preview/router.ex

@@ -10,6 +10,7 @@ defmodule BDS.Preview.Router do
   alias BDS.Posts.Post
   alias BDS.Posts.Translation
   alias BDS.Rendering
+  alias BDS.Rendering.TemplateSelection
   alias BDS.Repo
 
   @type route ::
@@ -219,7 +220,9 @@ defmodule BDS.Preview.Router do
       _post_record: effective_record
     }
 
-    case Rendering.render_post_page(project_id, post.template_slug, assigns) do
+    effective_slug = post.template_slug || TemplateSelection.resolve_post_template_slug(project_id, post.tags, post.categories)
+
+    case Rendering.render_post_page(project_id, effective_slug, assigns) do
       {:ok, rendered} -> {:ok, rendered}
       {:error, _reason} -> {:error, :not_found}
     end

lib/bds/rendering/template_selection.ex

@@ -4,13 +4,52 @@ defmodule BDS.Rendering.TemplateSelection do
   import Ecto.Query
 
   alias BDS.Frontmatter
+  alias BDS.Metadata
   alias BDS.Projects
   alias BDS.Rendering.FileSystem
   alias BDS.Rendering.Filters
   alias BDS.Repo
   alias BDS.StarterTemplates
+  alias BDS.Tags.Tag
   alias BDS.Templates.Template
 
+  @spec resolve_post_template_slug(String.t(), [String.t()], [String.t()]) ::
+          String.t() | nil
+  def resolve_post_template_slug(project_id, tag_names, category_names) do
+    resolve_from_tags(project_id, tag_names) ||
+      resolve_from_categories(project_id, category_names)
+  end
+
+  defp resolve_from_tags(_project_id, []), do: nil
+
+  defp resolve_from_tags(project_id, tag_names) do
+    Repo.all(
+      from tag in Tag,
+        where:
+          tag.project_id == ^project_id and
+            tag.name in ^tag_names and
+            not is_nil(tag.post_template_slug) and
+            tag.post_template_slug != "",
+        select: tag.post_template_slug,
+        limit: 1
+    )
+    |> List.first()
+  end
+
+  defp resolve_from_categories(_project_id, []), do: nil
+
+  defp resolve_from_categories(project_id, category_names) do
+    {:ok, state} = Metadata.get_project_metadata(project_id)
+    settings = state.category_settings || %{}
+
+    Enum.find_value(category_names, fn cat_name ->
+      case Map.get(settings, cat_name) do
+        %{"post_template_slug" => slug} when is_binary(slug) and slug != "" -> slug
+        _ -> nil
+      end
+    end)
+  end
+
   @spec load_template_source(String.t(), atom(), String.t() | nil) ::
           {:ok, String.t()} | {:error, term()}
   def load_template_source(project_id, kind, slug) do