Add CSS import/link/list/delete/content API (ticket 010)

Wires up Settings.css_entries behind a small settings API: import raw CSS
content to a file stored alongside settings.json, link external CSS
paths/URLs by reference (never deletable/copied), list all entries, delete
imported entries, and serve back imported content. CSS storage logic lives
on Settings (import_css/link_css/remove_css/css_content) with api.rs
handlers staying thin, matching the existing engine-query pattern.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/dto.rs

@@ -1,4 +1,5 @@
 use crate::data_value::MetricPrimitiveKind;
+use crate::settings::CssEntry;
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize)]
@@ -45,3 +46,58 @@ pub struct TimelineParams {
     pub end_ms: Option<i64>,
     pub bucket_ms: Option<i64>,
 }
+
+/// Request body for `POST /api/settings/css`: raw CSS content to import and
+/// store alongside the settings file.
+#[derive(Debug, Deserialize)]
+pub struct ImportCssRequest {
+    pub filename: String,
+    pub content: String,
+}
+
+/// Request body for `POST /api/settings/css/link`: a reference to an
+/// external CSS path/URL that ottyl doesn't copy or own.
+#[derive(Debug, Deserialize)]
+pub struct LinkCssRequest {
+    pub path_or_url: String,
+}
+
+/// Response shape for a single CSS entry, used both by the list endpoint and
+/// by mutation endpoints that return the affected entry. `id` is the entry's
+/// real id for `Imported` entries, or a stable id derived from the
+/// linked path/URL for `Linked` entries (see `settings::css_entry_id`) —
+/// this lets the frontend key/reference both kinds of entries uniformly,
+/// even though only `Imported` entries are deletable/content-fetchable.
+#[derive(Debug, Clone, Serialize)]
+pub struct CssEntryDto {
+    pub id: String,
+    pub kind: CssEntryKind,
+    pub filename: Option<String>,
+    pub path_or_url: Option<String>,
+}
+
+#[derive(Debug, Clone, Copy, Serialize)]
+#[serde(rename_all = "lowercase")]
+pub enum CssEntryKind {
+    Imported,
+    Linked,
+}
+
+impl From<&CssEntry> for CssEntryDto {
+    fn from(entry: &CssEntry) -> Self {
+        match entry {
+            CssEntry::Imported { id, filename } => CssEntryDto {
+                id: id.clone(),
+                kind: CssEntryKind::Imported,
+                filename: Some(filename.clone()),
+                path_or_url: None,
+            },
+            CssEntry::Linked { path_or_url } => CssEntryDto {
+                id: crate::settings::css_entry_id(entry),
+                kind: CssEntryKind::Linked,
+                filename: None,
+                path_or_url: Some(path_or_url.clone()),
+            },
+        }
+    }
+}