feat(sprints): modèle de sprints — domaine, use-cases, persistance et surfaces (backend)

Ticket #10 — introduction du modèle de sprints côté backend.

- Domaine : nouvel agrégat Sprint (sprint.rs), IDs, événements et invariants ;
  rattachement des issues à un sprint (issue.rs) et ports associés.
- Application : use-cases sprints (application/src/sprints) + erreurs dédiées.
- Infrastructure : store de sprints (infrastructure/src/sprints.rs), adaptation
  du store d'issues et exposition MCP via orchestrator/mcp/tickets.rs.
- app-tauri : commandes, state et events pour piloter les sprints depuis l'UI.

Tests domaine/application/infra/app-tauri verts (sprint_usecases, sprint_store,
issue_store, mcp_server).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/domain/src/issue.rs

@@ -8,7 +8,7 @@ use std::str::FromStr;
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
 
-use crate::ids::{AgentId, IssueId};
+use crate::ids::{AgentId, IssueId, SprintId};
 use crate::markdown::MarkdownDoc;
 
 /// Sequential per-project issue number.
@@ -255,6 +255,8 @@ pub struct Issue {
     pub status: IssueStatus,
     /// Priority.
     pub priority: IssuePriority,
+    /// Sprint membership. `None` means backlog / no sprint.
+    pub sprint: Option<SprintId>,
     /// Editable issue-local knowledge.
     pub carnet: MarkdownDoc,
     /// Links to other issues.
@@ -299,6 +301,7 @@ impl Issue {
             description,
             status,
             priority,
+            sprint: None,
             carnet,
             links,
             agent_refs,
@@ -392,6 +395,8 @@ pub struct IssueIndexEntry {
     pub status: IssueStatus,
     /// Priority.
     pub priority: IssuePriority,
+    /// Sprint membership.
+    pub sprint: Option<SprintId>,
     /// Assigned agent ids.
     pub assigned_agent_ids: Vec<AgentId>,
     /// Last update time.
@@ -406,6 +411,7 @@ impl From<&Issue> for IssueIndexEntry {
             title: issue.title.clone(),
             status: issue.status,
             priority: issue.priority,
+            sprint: issue.sprint,
             assigned_agent_ids: issue
                 .agent_refs
                 .iter()
@@ -426,6 +432,8 @@ pub struct IssueListFilter {
     pub priority: Option<IssuePriority>,
     /// Optional assigned agent filter.
     pub assigned_agent_id: Option<AgentId>,
+    /// Optional sprint membership filter.
+    pub sprint: Option<SprintId>,
     /// Optional case-insensitive text query.
     pub text: Option<String>,
 }