feat(server): partage ciblé de programmes et séances entre comptes (ticket #51)

Ajoute l'endpoint api/share_api.dart, les use cases de partage
(application/share_use_cases.dart) et l'adapter Postgres
(infrastructure/postgres/share_repository.dart). L'acceptation d'un
partage crée une nouvelle ressource synced_resources avec de nouveaux
IDs pour le destinataire, sans jamais modifier la ressource source de
l'émetteur ; gestion des conflits révoqué/déjà répondu. dart pub get
OK, dart analyze clean, dart test 24/24 vert. Use case d'acceptation
relu manuellement et jugé correct ; pas de test de bout en bout contre
un vrai PostgreSQL faute d'accès Docker dans ce sandbox.

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

server/README.md

@@ -104,18 +104,34 @@ server clock if no resource is returned.
 `pushResults` included so per-item validation errors remain visible to the
 client.
 
+## Sharing
+
+Ticket #51 adds authenticated targeted sharing for programs and workout
+templates.
+
+Endpoints:
+
+- `POST /shares` with `Authorization: Bearer <token>`.
+- `GET /shares/inbox` with `Authorization: Bearer <token>`.
+- `POST /shares/{id}/accept` with `Authorization: Bearer <token>`.
+- `POST /shares/{id}/decline` with `Authorization: Bearer <token>`.
+- `POST /shares/{id}/revoke` with `Authorization: Bearer <token>`.
+
+Creating a share stores a snapshot payload and creates pending recipient rows
+for known recipient emails. Unknown emails are reported as `unresolvedEmails`
+without failing the whole request. Accepting a share creates a new
+`synced_resources` copy owned by the recipient with a server-generated
+`clientId`; the sender's original resource is never modified. Accept returns
+`409` for revoked or already answered shares, while missing shares or recipients
+return `404`.
+
 ## Scope
 
 Ticket #47 only scaffolds the Dart server, the hexagonal directory layout and
 the `/health` endpoint. Ticket #49 adds the first PostgreSQL schema. Ticket #48
-adds authentication. Ticket #50 adds sync only; sharing behavior is still
-implemented in a later ticket.
+adds authentication. Ticket #50 adds sync. Ticket #51 adds targeted sharing.
 
 Upcoming tickets will fill the empty adapters and use cases:
 
-- #48: user authentication and API tokens.
-- #49: PostgreSQL schema and repositories.
-- #50: incremental sync API.
-- #51: targeted sharing.
 - #52: Docker and registry packaging.
 - #53: API, contract and integration tests.