feat: Einzelne Nachrichten als Link teilen

Backend:
- SharedMessageRecord Model + shared_messages DB-Tabelle
- SharedEndpoint: POST /api/messages/{id}/share, GET /api/shared/{token},
  GET /api/shared (Liste), DELETE /api/shared/{token}
- Auth-Ausnahme für GET /api/shared/{token} (öffentlich ohne Login)
- DB-Methoden: CreateSharedMessage, GetSharedMessage, GetChainMessagesUpTo

Frontend:
- Share-Button (fa-share-alt) an jeder Nachricht
- Kontextmenü: Einzelnachricht oder mit Thread-Kontext teilen
- Link wird automatisch in Zwischenablage kopiert
- Öffentliche Shared-Page (#shared/{token}) ohne Login
- Markdown-Rendering in der geteilten Ansicht
- CSS für Share-Button, Share-Menü und Shared-Page

Services/DatabaseService.cs

@@ -228,6 +228,20 @@ public class DatabaseService
             CREATE INDEX IF NOT EXISTS idx_cronjobs_user ON cronjobs(user_id);
             CREATE INDEX IF NOT EXISTS idx_cronjobs_next_run ON cronjobs(next_run_at);
         ");
+
+        conn.Execute(@"
+            CREATE TABLE IF NOT EXISTS shared_messages (
+                id               INTEGER PRIMARY KEY AUTOINCREMENT,
+                share_token      TEXT NOT NULL UNIQUE,
+                message_id       INTEGER NOT NULL,
+                user_id          TEXT NOT NULL,
+                include_context  INTEGER NOT NULL DEFAULT 0,
+                expires_at       TEXT,
+                created_at       TEXT NOT NULL
+            );
+            CREATE INDEX IF NOT EXISTS idx_shared_messages_token ON shared_messages(share_token);
+            CREATE INDEX IF NOT EXISTS idx_shared_messages_message ON shared_messages(message_id);
+        ");
     }
 
     private void TryAddColumn(SqliteConnection conn, string table, string column, string type)
@@ -236,6 +250,65 @@ public class DatabaseService
         catch { /* column already exists */ }
     }
 
+    // ── Shared Messages ──
+
+    public async Task<SharedMessageRecord> CreateSharedMessage(long messageId, string userId, bool includeContext, string expiresAt = null)
+    {
+        using var conn = CreateConnection();
+        var record = new SharedMessageRecord
+        {
+            ShareToken = Guid.NewGuid().ToString("N"),
+            MessageId = messageId,
+            UserId = userId,
+            IncludeContext = includeContext,
+            ExpiresAt = expiresAt,
+            CreatedAt = DateTime.UtcNow.ToString("o")
+        };
+        await conn.ExecuteAsync(@"
+            INSERT INTO shared_messages (share_token, message_id, user_id, include_context, expires_at, created_at)
+            VALUES (@ShareToken, @MessageId, @UserId, @IncludeContext, @ExpiresAt, @CreatedAt)
+        ", record);
+        return record;
+    }
+
+    public async Task<SharedMessageRecord> GetSharedMessage(string shareToken)
+    {
+        using var conn = CreateConnection();
+        return await conn.QuerySingleOrDefaultAsync<SharedMessageRecord>(
+            "SELECT * FROM shared_messages WHERE share_token = @shareToken", new { shareToken });
+    }
+
+    public async Task<IEnumerable<SharedMessageRecord>> GetSharedMessagesByUser(string userId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QueryAsync<SharedMessageRecord>(
+            "SELECT * FROM shared_messages WHERE user_id = @userId ORDER BY created_at DESC", new { userId });
+    }
+
+    public async Task DeleteSharedMessage(string shareToken, string userId)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "DELETE FROM shared_messages WHERE share_token = @shareToken AND user_id = @userId",
+            new { shareToken, userId });
+    }
+
+    public async Task<MessageRecord> GetMessage(long messageId)
+    {
+        using var conn = CreateConnection();
+        return await conn.QuerySingleOrDefaultAsync<MessageRecord>(
+            "SELECT * FROM messages WHERE id = @messageId", new { messageId });
+    }
+
+    public async Task<List<MessageRecord>> GetChainMessagesUpTo(string chainId, long messageId)
+    {
+        using var conn = CreateConnection();
+        var result = await conn.QueryAsync<MessageRecord>(
+            "SELECT * FROM messages WHERE chain_id = @chainId AND id <= @messageId ORDER BY id ASC",
+            new { chainId, messageId });
+        return result.AsList();
+    }
+
     // ── Agents ──
 
     public async Task<AgentRecord> CreateAgent(AgentRecord agent)