feat(backend): implémentation complète du système de messagerie

Ajoute l'infrastructure backend complète pour les conversations et messages :

## Entités
- Conversation : conversations individuelles, groupes, broadcast, annonces
- Message : messages avec statut, priorité, pièces jointes, soft delete

## Repositories
- ConversationRepository : findByParticipant, findByIdAndParticipant (sécurité)
- MessageRepository : findByConversation, countUnread, markAsRead

## Services
- ConversationService : CRUD conversations, archive, mute, pin
- MessageService : send, edit, delete, getMessages

## REST Endpoints (12 total)
- GET /api/conversations - Lister mes conversations
- GET /api/conversations/{id} - Récupérer une conversation
- POST /api/conversations - Créer conversation
- PUT /api/conversations/{id}/archive - Archiver
- PUT /api/conversations/{id}/mark-read - Marquer comme lu
- PUT /api/conversations/{id}/toggle-mute - Activer/désactiver son
- PUT /api/conversations/{id}/toggle-pin - Épingler
- GET /api/messages?conversationId=X - Lister messages
- POST /api/messages - Envoyer message
- PUT /api/messages/{id} - Éditer message
- DELETE /api/messages/{id} - Supprimer message

## Database
- Migration V6 : tables conversations, messages, conversation_participants
- Indexes sur organisation, type, archived, deleted pour performance

## Sécurité
- SecuriteHelper.resolveMembreId() : résolution membre depuis JWT
- Vérification accès conversation avant toute opération
- @RolesAllowed sur tous les endpoints

Débloquer la fonctionnalité Communication mobile (actuellement 100% stubs).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/main/resources/db/migration/V6__Create_Communication_Tables.sql

@@ -0,0 +1,109 @@
+-- ============================================================================
+-- Migration V6: Tables Communication (Conversations + Messages)
+-- Date: 2026-03-16
+-- Description: Création du système de messagerie UnionFlow
+-- ============================================================================
+
+-- Table conversations
+CREATE TABLE conversations (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    name VARCHAR(255) NOT NULL,
+    description VARCHAR(1000),
+    type VARCHAR(20) NOT NULL CHECK (type IN ('INDIVIDUAL', 'GROUP', 'BROADCAST', 'ANNOUNCEMENT')),
+    organisation_id UUID,
+    avatar_url VARCHAR(500),
+    is_muted BOOLEAN NOT NULL DEFAULT FALSE,
+    is_pinned BOOLEAN NOT NULL DEFAULT FALSE,
+    is_archived BOOLEAN NOT NULL DEFAULT FALSE,
+    metadata TEXT,
+    updated_at TIMESTAMP,
+
+    -- Colonnes BaseEntity
+    date_creation TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    date_modification TIMESTAMP,
+    cree_par VARCHAR(255),
+    modifie_par VARCHAR(255),
+    actif BOOLEAN DEFAULT TRUE,
+    version INTEGER NOT NULL DEFAULT 0,
+
+    CONSTRAINT fk_conversation_organisation FOREIGN KEY (organisation_id)
+        REFERENCES organisations(id) ON DELETE SET NULL
+);
+
+-- Index pour conversations
+CREATE INDEX idx_conversation_organisation ON conversations(organisation_id);
+CREATE INDEX idx_conversation_type ON conversations(type);
+CREATE INDEX idx_conversation_archived ON conversations(is_archived);
+CREATE INDEX idx_conversation_created ON conversations(date_creation);
+
+-- Table de jointure conversation_participants (many-to-many)
+CREATE TABLE conversation_participants (
+    conversation_id UUID NOT NULL,
+    membre_id UUID NOT NULL,
+    joined_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    PRIMARY KEY (conversation_id, membre_id),
+
+    CONSTRAINT fk_conv_participant_conversation FOREIGN KEY (conversation_id)
+        REFERENCES conversations(id) ON DELETE CASCADE,
+    CONSTRAINT fk_conv_participant_membre FOREIGN KEY (membre_id)
+        REFERENCES membres(id) ON DELETE CASCADE
+);
+
+-- Index pour conversation_participants
+CREATE INDEX idx_conv_participants_membre ON conversation_participants(membre_id);
+CREATE INDEX idx_conv_participants_conversation ON conversation_participants(conversation_id);
+
+-- Table messages
+CREATE TABLE messages (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    conversation_id UUID NOT NULL,
+    sender_id UUID NOT NULL,
+    sender_name VARCHAR(255) NOT NULL,
+    sender_avatar VARCHAR(500),
+    content TEXT NOT NULL,
+    type VARCHAR(20) NOT NULL CHECK (type IN ('INDIVIDUAL', 'BROADCAST', 'TARGETED', 'SYSTEM')),
+    status VARCHAR(20) NOT NULL CHECK (status IN ('SENT', 'DELIVERED', 'READ', 'FAILED')),
+    priority VARCHAR(20) NOT NULL DEFAULT 'NORMAL' CHECK (priority IN ('NORMAL', 'HIGH', 'URGENT')),
+    recipient_ids VARCHAR(2000),
+    recipient_roles VARCHAR(500),
+    organisation_id UUID,
+    read_at TIMESTAMP,
+    metadata TEXT,
+    attachments VARCHAR(2000),
+    is_edited BOOLEAN NOT NULL DEFAULT FALSE,
+    edited_at TIMESTAMP,
+    is_deleted BOOLEAN NOT NULL DEFAULT FALSE,
+
+    -- Colonnes BaseEntity
+    date_creation TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    date_modification TIMESTAMP,
+    cree_par VARCHAR(255),
+    modifie_par VARCHAR(255),
+    actif BOOLEAN DEFAULT TRUE,
+    version INTEGER NOT NULL DEFAULT 0,
+
+    CONSTRAINT fk_message_conversation FOREIGN KEY (conversation_id)
+        REFERENCES conversations(id) ON DELETE CASCADE,
+    CONSTRAINT fk_message_sender FOREIGN KEY (sender_id)
+        REFERENCES membres(id) ON DELETE SET NULL,
+    CONSTRAINT fk_message_organisation FOREIGN KEY (organisation_id)
+        REFERENCES organisations(id) ON DELETE SET NULL
+);
+
+-- Index pour messages
+CREATE INDEX idx_message_conversation ON messages(conversation_id);
+CREATE INDEX idx_message_sender ON messages(sender_id);
+CREATE INDEX idx_message_organisation ON messages(organisation_id);
+CREATE INDEX idx_message_status ON messages(status);
+CREATE INDEX idx_message_created ON messages(date_creation);
+CREATE INDEX idx_message_deleted ON messages(is_deleted);
+
+-- Commentaires
+COMMENT ON TABLE conversations IS 'Conversations (fils de discussion) du système de messagerie';
+COMMENT ON TABLE conversation_participants IS 'Association many-to-many entre conversations et membres participants';
+COMMENT ON TABLE messages IS 'Messages individuels dans les conversations';
+
+COMMENT ON COLUMN conversations.type IS 'Type: INDIVIDUAL (1-1), GROUP (groupe), BROADCAST (diffusion), ANNOUNCEMENT (annonces)';
+COMMENT ON COLUMN messages.status IS 'Statut: SENT (envoyé), DELIVERED (livré), READ (lu), FAILED (échec)';
+COMMENT ON COLUMN messages.priority IS 'Priorité: NORMAL, HIGH (élevée), URGENT (urgente)';