feat(api): reshape raw events into TimelineItem

GET /v1/events now returns the presentation form rather than raw
upstream payloads. The frontend stays dumb: it renders title /
subtitle / body segments and picks an icon from a small kebab-case
enum. Title and subtitle are arrays of {text} | {text, url} segments
so the UI can interleave plain copy with anchors without parsing.

New entities (in moments-entities):

  TimelineItem        — id, source, action, occurred_at, icon, title, subtitle, body
  TitleSegment        — Text | Link
  TimelineBody        — Markdown | Commits | Links
  CommitSummary       — sha, short_sha, message, url, author
  TimelineIcon        — kebab-case enum; UI falls back to Generic on unknowns

Reshape lives in moments-core::presentation, dispatched by source.
github.rs covers the event types observed on grenade's feed:
PushEvent, PullRequest{,Review,ReviewComment}Event, Issues{,Comment}Event,
Create/Delete/Fork/Watch/Release/CommitComment/PublicEvent.
Anything else falls back to a generic "<action> on <repo>" line.
Other sources (gitea, hg, bugzilla) currently use a stub fallback;
they get their own reshape modules in steps 5 and 6.

4 unit tests cover the load-bearing cases (push commit list, merged
PR icon swap, issue-comment markdown body, unknown-event fallback).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/moments-entities/src/lib.rs

@@ -74,3 +74,79 @@ pub struct SourceSummary {
     pub earliest: Option<DateTime<Utc>>,
     pub latest: Option<DateTime<Utc>>,
 }
+
+// ---------------------------------------------------------------------
+// Presentation shape — what `GET /v1/events` actually returns.
+// The API reshapes raw payloads into these so the frontend stays dumb.
+// ---------------------------------------------------------------------
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct TimelineItem {
+    pub id: String,
+    pub source: Source,
+    pub action: String,
+    pub occurred_at: DateTime<Utc>,
+    pub icon: TimelineIcon,
+    /// Primary headline. Mixed plain text + inline links so the UI can
+    /// render the right anchors without parsing.
+    pub title: Vec<TitleSegment>,
+    pub subtitle: Option<Vec<TitleSegment>>,
+    pub body: Option<TimelineBody>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(tag = "kind", rename_all = "lowercase")]
+pub enum TitleSegment {
+    Text { text: String },
+    Link { text: String, url: String },
+}
+
+impl TitleSegment {
+    pub fn text(s: impl Into<String>) -> Self {
+        Self::Text { text: s.into() }
+    }
+    pub fn link(text: impl Into<String>, url: impl Into<String>) -> Self {
+        Self::Link {
+            text: text.into(),
+            url: url.into(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "kind", rename_all = "lowercase")]
+pub enum TimelineBody {
+    Markdown { text: String },
+    Commits { commits: Vec<CommitSummary> },
+    Links { items: Vec<TitleSegment> },
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CommitSummary {
+    pub sha: String,
+    pub short_sha: String,
+    pub message: String,
+    pub url: String,
+    pub author: Option<String>,
+}
+
+/// UI icon hint. The frontend maps these to its own icon set; new variants
+/// here require a frontend update but never break existing renders (the UI
+/// falls back to the generic icon for unknown values).
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "kebab-case")]
+pub enum TimelineIcon {
+    GitPush,
+    GitCommit,
+    GitMerge,
+    GitFork,
+    GitBranchCreate,
+    GitBranchDelete,
+    PullRequest,
+    Issue,
+    Comment,
+    Star,
+    Release,
+    Bug,
+    Generic,
+}